Sri Lanka and South Indian food share a lot in common, and many local restaurants will describe their menus as Sri Lankan & South Indian. There are a number of regional variations though, the different types of hopper, devilled prawns/cuttlefish/chicken/etc. and the common fiery addition to any curry, pol sambol made of grated coconut, red chilli powder and lime juice.
The food is very cheap generally, with a cheap meal costing about a dollar. The most expensive, tourist-orientated places seldom charge more than ten dollars. The staple food of Sri Lankans is rice and curry - a massive mound of rice surrounded by various curries and delicacies. If you want to eat a cheap lunch you can follow the Sri Lankan crowds and duck into any of a million small cafes, confusingly called 'Hotels'. These normally sell a rice and curry packet, as well as 'short eats', a collection of spicy rolls. This is ideal for backpackers and those who want to get past the touristy hotels selling burnt chicken and chips - you're charged by how much you eat, and unless you're absolutely ravenous it probably won't cost over a dollar.
If you are taking road trips outside Colombo, there are endless options for places to stop on the road for lunch. Rest houses and hotels along major roads throughout Sri Lanka have good restaurants, and will offer both Sri Lankan and Western menus. If you are less adventurous, you can easily get good sandwiches and soups at these restaurants. These places have excellent rice and curry plates, and you will be served many different types of curries over an extremely generous portion of rice. These meals are extremely delicious and will leave you full and happy at the end of the meal. Eating will definitely be a memorable experience in Sri Lanka.
Kottu Kothu Roti a medley of chopped roti, vegetables and your choice of meat is a must-have for anyone - tourist or otherwise - in Sri Lanka. It is uniquely Sri Lankan and tastes best when made fresh by street vendors. However, several kottu roti restaurants have been closed down due to their use of stale and old roti, which made some patrons sick. Use caution, and even better, talk with the locals to figure out where the best kottu roti restaurants are.
Sri Lankan food is generally spicy. But you can always ask for less spicy options when you are ordering your food.
Other food that you should try are String Hoppers , Hoppers, Pittu & Kiribath.
Note that Sri Lankans eat with their right hands - this isn't a major problem, because everywhere will be able to provide cutlery if you can't eat otherwise. But try the Sri Lankan way tips of fingers only!, it's harder than it looks but strangely liberating.
There are many upscale restaurants to choose from in the city of Colombo. There are several fine dining restaurants at the 5 star hotels which offer both Local and International cuisine. These establishments are found largely in western Colombo along Galle Road, though more are located around Colombo and other major cities.
Fast-food outlets such as KFC, Pizza Hut etc. can be found in major cities.