In general there are two main schools of Buddhism. Theravadin Buddhism is in Thailand, Cambodia, Laos, Burma and Sri Lanka. It's in South East Asia. The other school of Buddhism is called Mahayana Buddhism, which includes within it some very distinct styles. Mahayana is the Tibetan Buddhism, the Chinese Buddhism, the Japanese Zen, the Korean Zen, Vietnam type Zen. Those are all in the Mahayana school, but they splintered up into many different types.
Theravadin Buddhism has not really split up in the same way. The monks here in Thailand, they still do similar things as in Sri Lanka, Burma, etc., such as going on Alms round and keeping the same scriptures. Now as to all the differences between the two, it's far too much to discuss here, but if it's important to you, read about it. As to the main differences, Theravadin Buddhism is very down to earth, very practical. The Mahayana schools use a lot of belief systems. For example, the Theravadin Buddhism has the Kalama Sutta - you do not have to believe the teacher. The other schools do not have this teaching, they do not use it, so they do believe the teacher. And in some of them you're almost forced to believe the teacher is enlightened, you must follow everything the teacher says and so on. They very much have belief systems incorporated into it. They also believe the Buddha is still alive somewhere, in some heaven-type realm. Theravadin Buddhism says, "Wait a minute, enlightenment means you're finished, done." There are more philosophical and practice differences between the two schools.
Theravadin Buddhism is also the only Buddhism in which people can be enlightened here and now. Now that may sound a little weird but according to definition the Mahayana Buddhists all become Bodhisattvas. A Bodhisattvas cannot become enlightened. A Bodhisattva by definition, for those of you who don't know, is a being who is going to work possibly hundreds of lifetimes in order to become a Buddha one day in the future. So because they're a Bodhisattva it is impossible for them to be partly or fully enlightened already. In Theravadin Buddhism we can become partly or fully enlightened. So that's another big difference. It's impossible for anyone in the Mahayana tradition, as much as they praise all their teachers, to be enlightened by their own definition.