Buddhism is a fairly broad and complicated subject to summarize in a web page, but let's see if I can get you started. Buddhism evolved in the sixth to fifth century BC (see the history of Buddhism) based on the teachings of Siddhartha Gautama, a prince who became addressed as "Buddha" - enlightened one - after leaving the palace in search of meaning. Siddhartha's epiphany was that life's suffering - including the endless cycle of rebirth that was part of the beliefs of the people at the time - was caused by our very own attachment, or craving, for life, things, etc. By realizing this, and training our minds see the truth, we can release ourselves from all this suffering and become enlightened ourselves.
A Buddha Image from Sukhothai, one of the earliest Thai kingdoms.
While Buddhism as practiced in Thailand, and much of Asia, has all the trappings of a religion, it can just as accurately be seen as a philosophy - one that can supplant traditional religion. Indeed, much of what you may observe in Thailand's Buddhist temples may have very little to do with Buddhism, and actually reflects the pre-Buddhist animist beliefs of the inhabitants of Southeast Asia.
Buddhist Iconography
Speaking of what you'll observe in Buddhist temples, we may as well talk about Buddhist iconography; in other words, all the Buddha images. With all the images that people pray to, it's no wonder that Buddhism was labeled as idolatry by westerners. While this perception is perfectly understandable, I don't think it's entirely accurate. Buddha images are not meant to represent the Buddha himself, but rather to convey one or more of the lessons of the Buddhist canon. There are some 43 recognized poses for Buddha images, each with its own story.
Buddhist imagery evolved over time. In the beginning there were no statues or other images of the Buddha. Nobody really knows what he looked like. There were a few symbols, such as the wheel and the lotus flower. You may be surprised to learn that we have the Greeks to thank for the introduction of statues illustrating the life and lessons of Buddha. When Alexander the Great conquered Bactria in Northern India in the third century BC, he left behind soldiers who founded several states modeled on the states of Greece. These states infused Greek visual representation into the oral traditions of the nascent Buddhist beliefs to create the first Buddha images. These first images were quite realistic, but over time Buddha images have become more and more symbolic.
Temples and the Monastic Tradition
There are something like 40,000 Buddhist temples in Thailand. Just about every village or big city neighborhood will have one. Temples often serve as community centers, and many have schools associated with them as well. The construction of temples is funded by the community, and businessmen will often donate large sums to temples when they encounter success, to "make merit".
Most Thai men will spend some time as a monk at least once in their life, usually before marriage. The time spent may be as little as three days, but will usually be more like two weeks. Younger boys may spend an entire rains retreat of three months as novices.