Clear noodles are made from mung beans. Many Thai people who are on a diet, substitute clear noodles for rice noodles.
Soak in hot, warm or cold water (depending on what you are making) before cooking. The hotter the water, the more water the noodles soak up and the softer they become and the stickier the outside surface. This leads to some rules of thumb about what kind of dish takes what kind of soaking. For casserole type dishes (where the water in the noodles can get baked out) soak in hot water. For re-cooked dishes, like Thai spring rolls, use warm water. For stir fried noodles, soak in cold water otherwise the noodles will be overly sticky. The noodles are done soaking when they become opaque and are soft (like cooked spaghetti). After the noodles are cooked they will become clear. High quality noodles should not keep on expanding if soaked in water or soup for a long time.