help with understanding buddhism...?

In short Theravadins believe that they practice most closely what the Buddha taught. They do not recognize later texts that Mahayana do. Theravadins also view the obtainment of enlightenment as personal and that laypersons are unlikely to become arhats.

Mahayana is more 'liberal' in the sense that it is open to all peoples, not just monks and nuns. Instead of becoming an arhat, the goal in Mahayana is to become a bodhisattva. In this way, bodhisattvas delay Nirvana to help others become enlightened as well.

Here is a link to a good chart on their differences and similarities:

http://www.religionfacts.com/buddhism/fastfacts/di...

Theravada is a very basic, relatively unaltered form of Buddhism; it is the closest to what the Buddha taught. Mahayana is the classification of other forms that have been expanded upon, like in Zen Buddhism. Sometimes I compare the concepts (very very loosely) as Theravada is to Mahayana what Catholicism is to Protestantism. Its not accurate, the Catholic Church is far from teaching and practicing only the basics, but the comparison is accurate as far as Catholicism sprouted Protestantism.

I put a link to a great comparison page.