Leo Gura of Actualized.org has a really great lecture on what intelligence is that sort of breaks the bounds of other previous explanations of intelligence I've heard.

It's available on youtube here: https://www.youtube.com/watch?v=bQSUu2CRRBE

I only post this because it's a direct answer to the question, What is intelligence, and I find it to be a rather deep explanation compared to many others.