Yi Peng is actually a separate festival celebrated by the Lanna people of Northern Thailand, however, it coincides with Loi Krathong and the two are now often considered the same. Colorful lanterns adorn houses and temples, while monks, locals, and tourist launch thin paper lanterns into the sky.
The sky lanterns -- known as khom loi -- are made from rice paper and are heated by a fuel disk. When done correctly, the large lanterns fly surprisingly high, and often look like fiery stars at their peak altitude. Messages, prayers, and wishes for good luck are written on the lanterns before launch.
Lanterns can be purchased nearly everywhere during Yi Peng and Loi Krathong; temples sell them to tourists as a way of generating money.
TIP: Keep your head up, as some lanterns come with a string of firecrackers attached to the bottom. The fireworks often go wrong and drop into unsuspecting crowds!