Most non-chain restaurants in Lithuania serve Lithuanian cuisine.
Every city and many towns have these traditional restaurants. They are also pretty much the only option for a roadside meal (save for buying up at gas stations). City center restaurants are generally more expensive and have outdoor seating in summer; further from the center the prices are lower but the restaurants are harder to locate. In smaller towns, there are fewer options so in weekends all the local restaurants might be booked up for weddings.
Another cheap Lithuanian cuisine option in cities is to use canteens available at factories and other large non-downtown businesses. Most of them welcome outsiders, although opening hours are limited (~10:00-17:00 weekdays only); look for words "kavinė" and "valgykla" written on windows of uninspiring Soviet buildings.
In theory valgykla is self-service, kavinė is either a cafeteria or a casual restaurant, restoranas is a more prestigious affair while baras is a drink-oriented edifice (but meals are also available).
Lithuanians eat their main dish in mid-day and some also have an evening out, leading to the common restaurant opening times at ~11:00 and closing times at ~22:00. There are few eating out possibilities after 23:00 and even fewer before 10:00 (as Lithuanians have their breakfast and morning tea/coffee at home).
Fast food in Lithuania (pizzas and burgers)
If you prefer international fast food to culinary adventures you may use McDonald's and its Finnish competitor Hesburger. There are a few Pizza Hut and KFC parlors. In general, the worldwide chain presence is low-key in Lithuania with none having more than 10 outlets (with the exception of Hesburger, but Hesburger is regional rather than a worldwide chain).
Certain local chains have been more successful. Pizzerias are extremely popular with at least 8 large local chains (Čili Pica, Pizza Jazz, CanCan Pizza, Mambo Pizza, Submarine Pizza, Pizza Express, Fokus Pica and Charlie Pizza) and many smaller ones that usually concentrate on cheaper and less tasty pizzas. You can order food home (or to hotel) from each of them. The main chains serve other food besides pizzas but usually, this is less tasty. Čili is the most popular; queues for a table are not uncommon there despite it being more expensive than many competitors. In some of the pizzerias, you may also get Italian pasta, but this is not always the case.
Even though Lithuanians love shopping malls food courts are rare (fast food chains commonly have their own tables). Most restaurants (even fast food ones) tend to offer a rather long menu.
International cuisines in Lithuania
Chinese cuisine is very popular in Lithuania with numerous non-chain restaurants in every city and some towns. While most restaurants are owned by ethnic Chinese the waiters are usually Lithuanians (unlike in the West where they are also Chinese).
Kebab parlors are even more popular and while this type of food was brought in by Turkish immigrants now this business is developed by Lithuanians as well (even the small towns now have kebab stalls). Usually bought at kiosks kebabs are meant to be eaten quickly.
This continues the tradition of great acceptance of Turkic fast foods into the mainstream society which began by the Tatar and Karaim cuisines. Their pasties with meat inside (kibins and čeburekai) have long been the sole fast food in Lithuania and may even be considered a part of the Lithuanian cuisine.
Caucasian cuisine (Georgian, Armenian, Azeri) was among the first foreign cuisines to establish itself in Lithuania as both Lithuania and the Caucasus were ruled by the Soviet Union until 1990. Now available in all the cities and some towns the Caucassian restaurants are more oriented towards an evening out.
Other international cuisines are less common, although you may find sushi parlors and Indian restaurants in the major cities.
Lebanese, Morrocan or other Arabic restaurants are hard to come by.
Vegetarian, vegan, halal and kosher meals
If you are a vegetarian you won't encounter problems. Bread and potatoes are widely used in the national cuisine with certain dishes, such as the Potato pancakes, lacking any meat. Moreover, vegetarianism became somewhat popular in Lithuania in the 2000s together with certain Asian religious practices. There are now specialized vegetarian and vegan restaurants as well in the main cities. A new main-city fad is the raw (uncooked) food.
Given small Muslim and Jewish communities dedicated halal and kosher restaurants are very hard to come by and limited to Vilnius and Kaunas.
Chain restaurants are more common in major cities although some of them, such as Čili or Hesburger, are available in some minor towns as well. They are most easily found at or near shopping malls although some are at city centers. While most chain restaurants serve foreign cuisine some (such as "Forto dvaras") specialize in Lithuanian cuisine.
There are some expensive fine dining restaurants in Lithuania mainly located at major city downtowns (especially Vilnius) and resorts.
The restaurant prices are significantly lower than in the Western Europe (for comparable offers). Drinks are cheaper by an order of magnitude. This includes drinks in shops, not only at restaurants.
Tipping has been introduced by first Lithuanian travelers to the West in the 1990s and now is very common in cities and resorts but rare in smaller towns. There is no customary rate: some will leave 10%, others believe 20 euro cents is always enough.
If you eat together with locals it will be the best if everyone pays for his own meals/drinks (ask for separate bills before ordering food). However, if the other party provided you free services (e.g. drove you around) it is polite to pay for their meals/drinks. The locals may offer to cover your bill as hosts, but you should decline once or twice. Local men may offer to pay for visiting women's meals/drinks but accepting may make them believe this was a date.
Smoking is not allowed inside restaurants.