The Nepali national meal is daal bhaat tarkaari. It is essentially spiced lentils poured over boiled rice, and served with tarkari: vegetables such as mustard greens, daikon radish, potatoes, green beans, tomatoes, cauliflower, cabbage, squash etc, cooked with spices. This is served in most Nepalese homes and teahouses, two meals a day at about 10 AM and 7 or 8 PM. If rice is scarce the grain part may be cornmeal mush called Ato, barley, or chapatis (whole wheat 'tortillas'). The meal may be accompanied by dahi (yogurt) and a small helping of ultra-spicy fresh chutney or achar (pickle).
Traditionally this meal is eaten with the right hand. Curried meat -- goat or chicken -- is an occasional luxury, and freshwater fish is often available near near lakes and rivers. Because Hindus hold cattle to be sacred, beef is forbidden. Buffalo and yak are eaten by some but considered too cow-like by others. Pork is eaten by some tribes, but not by upper-caste Hindus. And like in India, some communities and tribes are vegetarians and do not eat meat of any sort.
Outside the main morning and evening meals, a variety of snacks may be available. Tea, made with milk and sugar is certainly a pick-me-up. Corn may be heated and partially popped, although it really isn't popcorn. This is called "kha-ja", meaning "eat and run!" Rice may be heated and crushed into "chiura" resembling uncooked oatmeal that can be eaten with yogurt, hot milk and sugar, or other flavorings.
Fritters called 'pakora' and turnovers called "samosa" can sometimes be found, as can sweets made from sugar, milk, fried batter, sugar cane juice, etc. Be sure such delicacies are either freshly cooked or have been protected from flies. Otherwise flies land in the human waste that is everywhere in the streets, then on your food, and so you become a walking medical textbook of gastrological conditions.
Because of the multi-ethnic nature of Nepali society, differing degrees of adherence to Hindu dietary norms, and the extreme range of climates and microclimates throughout the country, different ethnic communities often have their own specialties.
Newars, an ethnic group originally living in the Kathmandu Valley, are connoisseurs of great foods who lament that feasting is their downfall (whereas sexual indulgence is said to be the downfall of Paharis). In the fertile Kathmandu and Pokhara valleys this cuisine often includes a greater variety of foodstuffs -- particularly vegetables -- than what are available in most of the hills. As such, Newari cuisine is quite distinct and diverse relatively compared to the other indigenous regional cuisines of Nepal, so watch out for Newari restaurants. Some of them even come with cultural shows...a great way to enjoy good food while having a crash-course in Nepalese culture.
The cuisine of the Terai lowlands is almost the same as in adjacent parts of India. Locally-grown tropical fruits are sold alongside subtropical and temperate temperate crops from the hills. In addition to bananas ('kera') and papayas ('mewa') familiar to travelers, jackfruit ('katar') is a local delicacy.
Some dishes, particularly in the Himalayan region, are Tibetan in origin and not at all spicy. Some dishes to look for include momos, a meat or vegetable filled dumpling (similar to Chinese pot-stickers) often served with beer, and Tibetan Bread and Honey a puffy fried bread with heavy raw honey that's great for breakfast. One delicacy that you do not want to miss while in Nepal is the dried meat (it especially complements with beer/alcoholic beverages. Up in the Himalayan mountains, potatoes are the staple of the Sherpa people. Try the local dish of potato pancakes (rikikul). They are delicious eaten straight off the griddle and covered with dzo (female yak) butter or cheese.
Pizza, Mexican, Thai and Chinese food, and Middle-Eastern food can all be found in the tourist districts of Kathmandu and Pokhara. If you are on a budget, sticking with local dishes will save a lot of money.
Note that many small restaurants are not prepared to cook several different dishes; try to stick with one or two dishes or you will find yourself waiting as the cook tries to make one after another on a one-burner stove.
As far as possible, eat only Nepali village products. Do not eat junk foods like biscuits, noodles etc. If you take only village product foods, it will help to rise their economic life.