There are many ways to slice and dice what learning and knowledge can be understood as. I'm no expert, but my daily work is in educating youth organisations (Scouting) and occasionally with training leaders for such organisations.

I'm in the school of Bloom's Taxonomy for learning objectives. It goes like this:

1. Remember

2. Understand

3. Apply

4. Analyse, evaluate and create

This grading goes from the known and concrete to the unknown and abstract. But, as said, there are many ways to look at this. Some say that the fourth point can be separated into three points, other's argue they are of equal level of skill. Just take a look at the Wikipedia article for Bloom's Taxonomy (http://en.wikipedia.org/wiki/Blooms_Taxonomy) and you'll see that it even portrays several different perspectives to one (actually two) theory.

Teaching something generally forces you to review what you know and organize your thoughts on the subject. It's not hard to see why that improves your understanding.