Rats can more less eat anything a human can eat. There are a few items on the prohibited list and it is very important to keep these items away from your rats, especially since rats cannot vomit.

Bad Foods
A lot of people say that chocolate is poisonous to rats and other people dispute that and say its fine in small doses. I'm not sure which is right but as a rule I keep chocolate away from my rats just to be on the safe side. Some other bad foods to keep away from rats include raw sweet potatoes, blue cheese, green potato skin and eyes, licorice, raw artichokes, raw brussel sprout, and raw red cabbage. Also be sure to keep carbonated drinks away from rats because rats can't burp!

Food mixes
Many people prefer to make their own food mix as opposed to purchasing food directly from a pet store. It can be cheaper to buy the foods in bulk and mix it yourself.

If you decide to make your own mix I'd recommend purchasing a plastic bin to put it in. Below is a picture of my old bin.

It is very important to do a lot of research into what exactly makes a good, healthy and balanced diet. I no longer make my own mix and I recommend purchasing a pre-made mix. I've used a few home made mixes over the years from a few different web sites and I've found them all to be too fatty. If you do decide to make your own please be sure to research it thoroughly.

In addition to their normal food, once a week I like to give them frozen or cooked vegetables and some cooked chicken or steak. They really enjoy it as a treat. Be sure to not to give them anything with spices on.

Treats
I give my rats yogurt drops as treats and they really enjoy them. The second they hear the sound of the bag opening they come running to the cage door. Giving rats treats like this are fine once in a while but just note that they contain no good nutritional value whatsoever! So no more than one or two yogurt drops a day. Even that might be pushing it depending on the size of your rats.