Today, like much of the world, Sri Lanka has adopted numerous international foods and preparation techniques. Pre-packaged foods are more common and numerous "ethnic" restaurants or fast food restaurants are present in many of the island's large cities. However, these changes haven't really changed the local foods, they have only added new foods to the diet and these foreign foods are rarely consumed by the locals so for many these outside influences have changed very little.
Staple Foods
Curry: any "wet dish" cooked in oil, can contain any combination of spices
Pulse/legumes: any bean, chickpea, or lentil dish, each of which act as a staple in various parts of India; dal and masoor are both forms of pulses
Rice: served as a base in many dishes and is prepared in numerous ways
Regional Variations, Specialties, & Unique Dishes
Hoppers: rice flour and coconut milk with various other ingredients
Koola'ya: mixture of curries served with rice; a leftover dish
Lamprais: curry cooked with meatballs and rice in a banana leaf
Dining Etiquette
Before eating with locals in Sri Lanka, know what religion your company is. The Hindis and Muslims have differing dining rules and habits so knowing what religion your host is will alter a couple etiquette rules. Fortunately, most of these rules pertain to foods that can or cannot be consumed and if eating in the home of a local they simply will not have foods that are forbidden by their religion. If dining out though, be sure to avoid ordering foods your local hosts won't eat. Hindis don't eat beef and many are vegetarian so never order beef if it's available. The Muslims don't eat pork or drink alcohol so again avoid these foods. To be safe, order chicken or fish and to be extremely cautious, order a vegetarian meal.
No matter the person's religion, you should dress conservatively, meaning your arms and legs should be covered and for women in the presence of Muslims the head should be covered as well. You should also arrive on time (although food might not be served for a couple hours) and take your shoes off at the door if others do so.
You will likely be asked to wash your hands prior to eating, but sometimes a wash basin is passed around the table so follow the lead of others. Let your host show you a seat, which may be on the floor. When seated, be sure to avoid pointing the bottom of your feet at anyone so keep your feet flat on the floor or pointed behind you.
You should try all the foods your host recommends and in some settings this may be a huge number of foods, but take much more rice than anything else as this is meant to be the base. You should also accept more food when offered so try your best to take limited quantities of food at first (a challenge on many occasions). If you take your own food, be sure to avoid touching your plate with the serving spoons as this is considered unclean. Once you have food you may notice there are no utensils (cutlery); this is because you are expected to eat with your right hand and right hand only as the left is considered unclean. You may use naan or rice to scoop the food or to soak up the sauce.
As you finish eating, leave some food on your plate to signify your host has provided more than enough food. You should again wash your hands after a meal and expect to be shown the door shortly after you are finished eating as socialization is generally limited to the time before a meal.
If dining in a restaurant check to see if there is a service charge already added to the bill, as it usually is in high end restaurants. If no service change is included, tip about 10%.
Celebrations & Events
Sri Lanka has few celebrations that essentially require particular foods to be served. At most festivals and celebrations the people tend to dine on national dishes or personal favorites. The one underlying commonality among these foods and celebrations is that most include numerous sweets and other desserts.
Drinks
Much of the tea the Indian subcontinent is famed for is grown in Sri Lanka and the island also boasts some of the world's best teas; it is definitely worth a try even if you aren't a tea drinker. Juice is another local favorite as numerous fruits are grown locally; passion fruit, mango, pineapple, and others are all available, as is coconut milk. Other internationally popular drinks including coffee and soft drinks are also easily available.
If you want an alcoholic beverage, try the local toddy, which is fermented from palm tree sap or arrack, which is a distilled spirit made from coconut or other local fruits, but be careful as this can be quite strong. Well-known international beers and liquors are also available in most hotels and resorts. A decent wine selection is somewhat limited to high end hotels, but still available.
The tap water in Sri Lanka is generally not safe so should not be consumed. Be sure to also avoid anything with ice as it may have been made from the tap water and some juices may have water added so be sure to ask prior to drinking the local juices. Salads and fruits could have also been washed in the tap water so be careful with those foods as well.