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Wild Norway rats are "neophobic", which is to say they are wary of new things, especially new foods. This is because they cannot vomit, so if they eat something poisonous or irritating they have no quick way to get rid of it. Therefore, when presented with a new food they only try a tiny bit, then wait for a few hours to see if they still feel OK before having any more.
The extent to which this is true of pet rats varies. Some take days or weeks to get used to a new food. Some dive face-first into anything you offer them.
Taste, and variety in taste, is an important part of a Norway rat's pleasure in life, so homogenised, laboratory-style rat pellets which remove this pleasure are rather cruel - as well as being potentially bad for their kidneys, as some brands are very high in protein. I've tried offering rat pellets to rats who weren't used to them, and they didn't even sniff them: that is, they did not recognise them as even potentially edible. At the other end of the spectrum most rats will eat peas or sweetcorn straight away even if they've never seen them before.
Normal diet is grain-mix plus titbits, but rats can eat anything that could be digested by a human or a dog, plus some that couldn't (e.g. candles, soap, leather and eyeshadow). If you can afford it, and provided your own diet is reasonably healthy, they will live very happily on whatever you're having, including small amounts of alcohol (about 1 teaspoon of beer or cider per rat, or half a teaspoon of cream-based liquer): but they do prefer some hard foods or bone or wood to gnaw, to help keep their teeth short. As they grind their food up before swallowing, they can safely be given chicken-bones.
If you have rats you need never throw leftover food away again: there's always somebody ready and eager to eat it. But be warned that if you let your rats out while you are eating they are likely to sit on your chest and help themselves from the fork as it goes past, or swipe food from your plate - or even park themselves in the middle of your plate and expect you to eat round them.
Unlike rabbits and guinea-pigs, rats make their own vitamin C and can live without fruit and vegetables, but they enjoy small amounts provided the fruit is ripe. Carrots, sweetcorn, avocado and peas are particularly popular. Too much fruit and veg., however, can upset the digestion: and they strongly dislike fruit which is unripe or naturally sharp.
Norway rats love a bit of meat (fish fingers are a particular favourite), but a diet which is very high in protein can cause itching skin, excessive urination, and possible kidney damage. Meat, nuts and high-protein dog biscuits should therefore be given only in small amounts.
As part-time predators, many rats will kill and eat smaller animals such as mice. Nearly all will eat insects, slugs and etc. if they can get at them.
Like mice, rats also love dairy products: cheese, milk and butter. This doesn't seem to do them any harm, but too much can make their urine smell strong. They are also very partial to eggs, whether cooked or raw, and will happily wade up to the armpits in egg-yolk.
Starchy foods such as rice and pasta are great favourites. Most rats like few things better than a dish full of long, tangly spaghetti, complete with a meat and tomato sauce.
They tend to be less enthusiastic about potato, though they will usually accept a chip (notice to American readers - I mean what you call French Fries - the things Americans call chips and the British call crisps would usually be too salty for them).
They also tend to like quite spicy food. They do have individual tastes, and some will e.g. refuse anything with garlic in, or eat only one flavour of chocolate drop - but on the whole fancy rats are very fond of a bit of Chinese cooking or a moderately hot curry.
Since their teeth are constantly renewed dental decay is not a problem, so rats can safely be given sweets, e.g. the sort of chocolate drops sold for dogs, guinea-pigs etc.. In fact I give mine the sort of penny-sized white chocolate discs covered in little multi-coloured sprinkles which are sold for children: they seem to enjoy crunching the sprinkles.
Be careful what you leave around when your rats are out. Not only will they eat anything a human would eat - they will also happily take up human vices. They will chew tobacco and inhale cannabis smoke and solvents (glue, shoe-whitener etc.), but these things can make them very ill, or even kill them.
They will also eat lead if they can get at it. I'm told it tastes sweet to them: but it can and will kill them, and if a nursing doe eats lead it will kill her babies as well. Keep them away from anything with lead in it, including toys containing lead weights, and old paint.
Many rats are fond of a little tea or coffee, preferably straight from the mug. Provided it's not too hot it doesn't seem to do them any harm.
Provided your rats are getting a reasonable amount of exercise - which they should be - it is usually OK to feed ad lib. An average adult rat probably eats roughly a heaped tablespoon of mixed grain a day, while growing youngsters and nursing does will need about twice as much: begin with that and then adjust the amount until there is a just a little left in the dish when you go to refill it, and then you know that they are getting all they need without undue wastage. Bear in mind, however, that nearly all commercial grain-mixes contain something very few rats will eat - the alfalfa pellets normally included in hamster-mix; the chicken-flavoured biscuits in Reggie Rat - and these things will be left uneaten in the bowl even if your rats are nearly starving. You need to give them enough for them to have left over a small amount of the food they actually consider edible.
However, some rats are very greedy and/or have a genetic predisposition to obesity, and if allowed to will eat until they resemble furry footballs. In very extreme cases, where the rat looks and feels solid like an overstuffed bolster, this can limit the rat's movement, causing problems washing and even getting up, and may even lead to an early death from heart-attack or stroke. Such extreme cases should be put on a diet and encouraged to exercise.
Some sources believe that even moderate obesity increases the frequency of tumours, but if so the effect is not very large, and tumours in rats are nearly all harmless - though expensive to remove. There seem to be no other adverse effects: indeed, provided they can still bend in the middle fat rats tend to be healthier, though this may be because unhealthy rats don't gain weight, rather than because heavy rats don't get sick.
There is one exception to this. A rat which is very noticeably overweight even as a small baby, with floppy, baggy fat which spreads out like the skirts of a hovercraft, may have the Zucker gene. Zucker rats are susceptible to a variety of health problems such as hypertension and a poor immune system, and generally die young. However, there is no point in worrying about how much they eat, since the fat on Zucker rats is fixed and cannot be shifted by dieting. Such animals have a major metabolic fault and will die whatever you do: so you might as well let them eat like horses and die happy.
Floppy, baggy fat of this kind which develops only in adulthood is not particularly sinister, however. I used to have a Himalayan buck called GBH (he was actually quite a nice animal: but played very rough as a kitten) who was so fat and floppy that when he walked outlying regions of rat literally swept the floor on either side. He lost the use of his hind-legs when he was about 2½ and after that he got a lot thinner, presumably due to having to work extra hard just to get about: but he lived to be nearly 3, making him one of the oldest bucks I've ever kept.
Unlike rabbits, Norway rats are not obliged to eat their own droppings as a matter of course: but some do so occasionally. It's a sort of double distillation: a way of extracting every scrap of goodness out of their food.
Norway rats need a great deal of water, and should never be left without it for more than a couple of hours, especially in warm weather. If you have to transport them under circumstances which make it difficult to give them a water-bottle or bowl - such as in a jolting vehicle - then they should be given moist food such as cucumber, grapes and juicy apple.
Many rats enjoy paddling up to their armpits in water, and so prefer an open water-bowl to a drinking bottle. Open water dishes should be placed in an area of the cage where they are not likely to be contaminated by woodshavings and droppings falling from a higher level: even so they may need changing twice a day. Some rats habitually bury all their dishes in woodshavings, and these have to have a bottle: place it so they can't get at the plastic cap to chew holes in it.
Even if the water-bottle is still quite full, change the water at least every two days and preferably every day, otherwise it will become stale and taste nasty. Beware of fancy bottles with very stiff, supposedly leak-proof mechanisms: they almost never are leak-proof, but may be hard for elderly rats to manage.
Another option is a device called a water-fountain, as sold by The Hatchwell Co. Ltd, Rishton, Blackburn, UK. This consists of two small, shallow plastic dishes joined together, with a channel connecting them, and a plastic tube, sealed at one end and open at the other and with a nick out of the open end. The tube is filled with water and one of the pair of dishes pushed over the open end, forming a tight seal, and then the whole thing is turned upright. The nick in the end of the tube allows water to flow out into the second bowl to the depth of the nick, which is less than the depth of the bowl: so you end up with a small bowl filled nearly to the top with water, and constantly topped up for as long as there is water in the tube. This is identical to the water-fountains sold for birds, except that the drinking-vessel part has been adapted to take a broad nose rather than a narrow beak.
These water-fountains are in my opinion preferable to bottles, except in a moving vehicle: like a bottle they can be placed inches above the floor so they don't get buried in shavings, but unlike a bottle they don't leak/drip, and are easy for the rat to drink from without turning its head nearly upside-down. The disadvantages are that they require both vertical bars to clip on to and a horizontal slot for the dish to stick into the cage through, so you will probably have to cut a slot in the bars for it and do quite a lot of adaptation - and some rats chew the edge of the dish. It's made of very tough plastic, so it takes them months to do it much damage; but eventually they'll probably gnaw the edge to below water-level and all the water will pour out and you'll have to get a new water-fountain. It would not, therefore, be a good idea to go away for the weekend and leave your rats with one of these fountains, in case they destroyed it the first night and then had no more water. Also watch out for cracks in the plastic tube, leading to leakage.