What is a KeyPoint and how is it determined?

I have been studying openCV for about a month now, and have been looking at the SIFT and SURF algorithms for "KeyPoint" detection.

My question is: What exactly is a KeyPoint? The openCV documentation uses the work "salient-feature" detection. What makes a feature a KeyPoint? and how is it determined? What are the applications of the SIFT/SURF or similar algorithms?