What are the main schools of Tibetan Buddhism?

Can anyone provide short (maybe a couple of sentences) summaries of the main schools of Tibetan Buddhism and the differences between them. In fairly certain there are 4 main schools but I could be wrong in that. Also if there are more minor schools of interested then a link to further information on those too would be really good. I've read a bit about Tibetan Buddhism but the characteristics and differences of the main schools just won't lodge in my mind so this would satisfy bit of personal interest really. Many thanks for all responses.