There are a lot of differences. The main difference is that Theravadin Buddhism is very down to earth, very practical. Very simple in its basic essence, easy to start doing, easy to start seeing results. Mahayana Buddhism is not so down to earth, it has a lot of theories, a lot of views, a lot of belief systems that are way beyond what Theravadin Buddhism has. Some of the belief systems are similar, some are not. Mahayana Buddhism, as much as I understand, believes that the Buddha was not a human being but actually a god who manifested in a human body. Theravadin Buddhism goes "no, no, the Buddha was a human being just like us." So that is a major difference between the two. Theravadin Buddhism says that you can get fully enlightened, it says that there is one Buddha and many, many thousands who became enlightened and their name is "Arahant", for those of you who don't know, and they are fully enlightened. Mahayana Buddhism says "no that is not possible, everyone must become a Buddha, must become a Buddha instead of being just beneath a Buddha as an Arahant."
There are quite a few differences in this aspect. To a certain degree Mahayana Buddhism stops people from actually going for enlightenment now, they have a kind of condition where "I am going to get enlightened in the future'". From a point of view of meditation practice, there is a lot of Ego in the sense of "I am going to be a great human being in the future" rather than just doing the work now. Theravadin Buddhism says do the work now.
There are a lot of differences, big and little, and to a certain extent people can grow in either practice, it is a personality thing - which one they like more than the other, for people who are very down to earth, Theravadin Buddhism fits. The Kalama Sutta, which you all know about, is essential to the Theravadin tradition. I don't believe it is used at all in the Mahayana tradition. There is the belief that you must follow your teacher regardless of what your teacher says, which on occasion has caused a lot of harm, especially for a lot of westerners. In the United States we personally know some meditators who have been harmed by their Mahayana teachers because they believed their teacher was enlightened, but their teacher was obviously not. People have died because of this, a lot of harm has come. Theravadin Buddhism has the Kalama Sutta and says, "No, no, don't believe just because the teacher says something."
As to one of the most significant differences between Theravadin and Mahayana Buddhism - if I had to choose one particular thing as a difference it would hinge on the fact that in Theravadin Buddhism you don't have to believe anything. Prove it. Prove it absolutely for yourself. Test it. See if it works. Don't blindly believe me, don't blindly believe Rosemary. If we give you some advice, try it out and see if it works but don't just believe us just because we say so. That is very important.