There is another most important difference not mentioned by Vajradhara. This is in regards to the root of all suffering (which is what Buddhism is all about). There are 3 main causes of suffering: craving (or greed), aversion (hatred) and ignorance. The Theravadas maintain that one should extinguish even the last drop of craving and aversion, and thus attain enlightenment. This is what they emphasise. The Mahayanas say it is ignorance which one should try to dispel. The Vajrayanas add to this by stating that the reason we are ignorant is because of our impure perceptions, and so, we should aim at purifying our perceptions. The goal is the same, the methods are different. Hope this helps.