Thailand has a very versatile kitchen where all tastes and facets are present. All dishes have one thing in common and that's that they're all spicy. The dishes vary by region but generally throughout Thailand rice is the main product in each dish. In itself that isn't surprising given that Thailand is one of the largest rice producers in Asia. Due to growing number of Chinese in Thailand you see more and more influences from China in the Thai cuisine.
When eating in Thailand you don't get a knife and fork (like in the west), but a fork and spoon. The dishes are mostly cut into small pieces, which makes the knife unnecessary. The fork is used to slide the food on the spoon and then you can eat it. If you are not used to the spicy food in Thailand and want to order less spicy, you can say (phonetic) mai pet to the waitor when you order. The kitchen will prepare prepare your dishes less spicy.
Vegetarians will have no problems in Thailand. While sometimes limited choices on the menu, makes it difficult to order vegetarian dishes in some countries, this is not the case in Thailand. In Thailand, there are a lot of vegetarian dishes. Those meals taste great and the quality is as good as any ordered dish.
In Thailand you cannot drink water from the tap, however it's no problem to brush your teeth with tap water. If you want to drink water, do so only from bottled mineral water which are for sale almost everywhere. Icecubes are no problem, they are made by different breweries and used throughout Thailand.
In Thailand you can easily eat well for low prices at the many street foodstands which can be found througout Thailand. In most foodstands they are serving traditional Thai dishes. If you are not sure whether food which is served is good, then an indication of fresh food are foodstands which are used by the local Thai.
If you don't like Thai food, then there are many Western restaurants where you can eat. Also large hotels in major cities have western dishes on the menu. Huge fastfood companies, such as Burger King, McDonalds and KFC, are present in the major cities and tourist locations. A luxury we do not have is that fastfood companies as Mc Donalds deliver in your hotel or other accommodations. However, you need to speak Thai to be able to order. Therefore it is wise to ask the receptionist of your guesthouse or hotel to place an order for you.