One of the first main differences that I understand, and I am not an expert on Mahayana, so some of this may not be perfectly correct, but from what I understand one of the first main differences is that in Theravadin Buddhism they don't talk about the end of time. It is not important, it is another bit of those leaves in the trees that don't help us any. In Mahayana Buddhism they say there will come a time when everyone becomes enlightened at the end of existence, everyone will get enlightened together. That is a big difference, but that is just off in the future, why do we have to worry about it - that is the Theravadin perspective - it is no big deal.
Another difference is that in Theravadin Buddhism the Buddha was a man, Siddhatta Gautama was a human being who was born, existed, died and he is gone, because he attained full enlightenment. He no longer exists. Mahayana Buddhism believes that he wasn't actually a man, he was in a sense, a god, a deva, that came to earth, manifested in a human form, did what he did to show humans what could be done, and when he died he still existed somewhere else. Whether he still does or not, I can't answer that.
In Mahayana Buddhism they have quite a lot of beliefs, and this is the biggest difference between Theravadin and Mahayana, is that Mahayana Buddhism has a lot of beliefs built into it, "You must believe this and that." Theravadin Buddhism says, "No, no, look to the Kalama Sutta. We have the Kalama Sutta, they don't." Because we have the Kalama Sutta. we are very down to earth, we can be very practical, we just do the work.
Interesting enough we have a small number of Mahayana Buddhist practitioners who like to come to us sometimes. They say that they really love the Mahayana practice, it is so flowery and exiting and their teachers are so charismatic and this and that, and they love that, but they come to us to do the real work. It is sad in its own way, really, but from the perspective of Theravadin Buddhism, Mahayana Buddhism is like a whole different world. In the same way that Christianity is a different world, Judaism is a different world, it is different. It has so many differences to Theravadin Buddhism that Theravadin Buddhism does not consider Mahayana monks to be actually Buddhist monks. It has such a different belief system.