Eggs have a somewhat unusual place in Thai food. At the curry stall, they're most likely to appear fried 'sunny-side up' as an optional add-on to your rice and curry selection. As a westerner, especially an American, I had always thought of fried eggs as strictly a breakfast dish. But in Thailand they're a common addition to rice dishes eaten for lunch or dinner.
Eggs on sale in a Bangkok market (the pink ones are salted eggs)
More rarely, you may see small stuffed 'omelets' on sale in the curry stall. These are made by scrambling eggs and then pouring them slowing into a wok while rolling the pan around so that the eggs forms a thin sheet which cooks quickly in the hot wok. Once the egg sets, a filling of ground pork, baby corn and other vegetables stir-fried in a chilli tomato sauce is spooned into the centre. The edges of the egg sheet are then folded over the filling to form the omelette.
Chicken eggs are used most of the time for cooking. The main exception is egg based sweets such as "golden threads". For these, duck eggs are preferred for their colour and higher protein content, which helps to hold the cooked desserts together.