What are the differences between leaf and non-leaf pages?

Question

I've been running some index usage reports, and I'm trying to get a definition of Leaf and Non-leaf. There seem to be both Leaf and Non-leaf Inserts, Updates, Deletes, Page Merges, and Page Allocations. I really don't know what it means, or if one is better than the other.

If someone could give a simple definition of each, and also explain why Leaf or Non-leaf matters, it would be appreciated!