Main difference - that depends on the point of view, but some of the differences are that Tibetan Buddhism is part of the Mahayana tradition. It's important to know that there are two main traditions: Theravadin Buddhism which is in Thailand, Burma, Sri Lanka, Cambodia and Laos. It's about 190 million people. Then there is Mahayana Buddhism which is in Tibet, China, Japan, and Korea. In Japan and Korea is what we normally call Zen, and within that there's also about a hundred million people.
Chinese Buddhism is another type of Buddhism different to the others although these three are all included in Mahayana. Chinese is a bit different and there is no telling how many people, it might be half a billion born Buddhists there. Then there is Tibetan Buddhism which is part of Mahayana which is only about 5 to 10 million people. That's about it. Depends on how many Westerners now have joined it. It's the smallest group of Buddhists around the world.
Yet if you ask an average Westerner who is the leader of all Buddhism, what do they answer? The Dalai Lama. No, he's only the leader of the smallest, littlest part of Buddhism. So why is he so famous? For political reasons, his country was over run by the Chinese in the 1950's. So that's just a bit of clarification, Tibetan is the smallest of all the four main branches of Buddhism, and it's actually very small.
Now differences between Theravadin and Mahayana in general, which includes all three, is that Theravadin is very down to earth, it's very practical. We also have the Kalama Sutta, which in itself is one of the biggest differences. We have the Kalama Sutta and they don't. We have a teaching that says, "You don't have to believe!" Who do we believe then? You have to find out for yourself. So when you see for yourself then you can believe. Very much down to earth, very practical, no blind belief is needed in Theravadin Buddhism. It does have theories, rebirth, heaven realms, hell realms, that's fine. Those are the theories but we don't have to believe them. Now, that's a major difference in a lot of the Mahayana traditions. Teachers in those traditions will often say, "You must believe this, this is what is". We don't have to believe, that's the big difference between the two schools.
Mahayana traditions also has a concept that everyone has to be a Bodhisattva, that no one can get enlightened. We all have to wait until the end of existence. Nobody can get enlightened. Now, think about that, they are stopping everyone getting enlightened, even the Dalai Lama, not enlightened. By their own definition, not anything that I am making up or what Theravadin Buddhism makes up, by their own definition none of them are enlightened.
Theravadin Buddhism has people enlightened and even the Mahayana traditions will say that people in Theravada are enlightened. Okay, now, where are you going to find the people? By definition you find them in the Theravadin tradition. This is interesting because many of the Mahayana teachers will criticize Theravadin Buddhism as not being good enough because they are not Bodhisattvas. They aren't saying that the Theravadins aren't enlightened, just that they are not good enough because they are not Bodhisattvas, who are not enlightened. If you can follow me, it's bizarre.
This is the main difference that in the Mahayana tradition there is nobody enlightened, in the Theravadin tradition they have been. Though whether they are in our life or not is not for me to answer, but they have been so that's another difference in the two main schools.
There are other differences:
Theravadin tradition follows the same clothing the Buddha wore. The monks follow the same basic rules that he laid down. They go for breakfast food every morning which is their main meal, often their only meal of the day. They follow the same tradition in that way as to what the Buddha actually did and that is historically true by historians, again that's not something they are making up. It's just what is known from history.
Whereas when Buddhism went North to Tibet, China, Japan - it went to places that were cold. They had to change clothing immediately. It was too cold. They had to change other customs, too. In India, the giving of alms, food, the giving of food, supporting monks and nuns, and things were traditionally very deeply ingrained. But in places like Japan, in particular, it was not ingrained and years and years ago they actually started forcing the monks to have to work, to have farms, to get married, things like that - so things changed when it went North, whereas Theravadin Buddhism has kept the Monkhood, in particular, very similar to exactly when the Buddha was alive. So that's just a little of the differences.