What are the differences between leaf and non-leaf pages? 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!
Think about it like this: leaf level pages are the end of the road for the data search through the B-tree structure of an index. They contain the data defined in the index definition (or in the case of a clustered index, all of the table's data) and a row locator to the data row (in the case of a nonclustered index).

Non-leaf level pages contain the "road map" to the leaf level pages/data by including the key value as well as a pointer to either another non-leaf level page (depending on the B-tree depth and location of the intermediate page) or the resulting leaf level page (index page for a nonclustered index, and data page for a clustered index).

Edit:  Here's a good image to put a picture to the idea.