The most universal Malian dish is rice with sauce (often peanut "tiga diga na," tomato/onion/oil, or leaf/okra based - usually with some fish or meat if purchased or prepared for guests). "To," a gelatinous corn or millet food served with sauce, is another Malian classic, though more a village food than something most tourists would encounter. In the north, couscous is also quite common.
In the largest cities, decent "western" restaurants can be found, charging near western prices. Bamako even has good Chinese, Vietnamese, Italian, Lebanese and more. In smaller places, the standard Malian restaurant serves chicken or beef with fries and/or salad - usually edible and affordable, but boring and not particularly Malian. The better places in the more touristy areas may also have some local specialities. "Street food" is a lot more fun (and super cheap) - breakfast will be omelet sandwiches, lunch is usually rice with a couple sauces to choose from, and dinner presents many options including beans, spaghetti cooked in oil and a little tomato, potatoes, fried rice, chicken, meatballs, beef kebabs, fish, and salad. You can find little tables along the roadsides and near transport centers.
Snacks you may find for sale include little cakes (especially in bus stations), various fried doughs (either sweet or with hot sauce), peanuts, roasted corn if in season, sesame sticks, and frozen juices in little plastic sacks. Fresh fruit is widely available and always delicious. Some of the best are mangoes, papaya, watermelon, guavas, bananas and oranges - the particular selection depends on the season.
Of course, as in any tropical, underdeveloped country, food borne disease is a major concern for the traveller. The main culprits for diarrhea are untreated water (especially in rural areas) and fruits and vegetables which have not been peeled or soaked in bleach water - salads (even in fancy restaurants!) are likely to cause problems. You should also be sure any food (especially meat) is thoroughly cooked - generally more of a problem with Western food in restaurants than with Malian foods (which are usually cooked for hours). Drink bottled water, and talk to your doctor about bringing an antibiotic like cipro to treat diarrhea that is severe or does not improve over a couple days.
Treat tap water with suspicion. It is often so heavily chlorinated that one suspects few bugs could possibly survive in it. But short-term visitors will be safer with bottled water. There are several cheap local brands, but be warned that they are only drunk by foreigners and wealthy Malians: don't rely on finding bottled water in shops patronised by "ordinary" Malians. Soft drinks such as Coca-Cola or Fanta are more widely available and safe. But remember that Coke will make you want to go to the toilet, and so may leave you more dehydrated than before you drank it - a serious problem in this stunningly hot country. Street vendors sell water and home-made ginger and berry drinks in little plastic bags. They are often iced which makes them very refreshing in the heat. Generally, you shouldn't drink these without treating them first.
However, one which is called "bissap" in French and "dabileni" ("red hybiscus") in Bambara, is made from hibiscus flowers that are boiled during preparation, and so generally is safe to drink. It is a particularly delicious non-alcoholic drink you shouldn't miss. In Bamako, it is possible to purchase at most corner stores treated water in small plastic bags for XOF50; these are much cheaper, and of course more environmentally friendly, than bottles. The bags are marked with a brand name; be careful not to mistake them for the tap water that is sold in unmarked plastic bags by street vendors. Also widely sold in this way is sweet milk and yoghurt, which are normally clean because the bags are industrially filled. Fresh milk can also be bought from buckets at the roadside in some villages, although it should always be thoroughly boiled before drinking as it can carry tuberculosis bacteria (often Malians do this before selling, but it is safer to do it yourself or at least ask).