What we meant by a 100% crop?
A 100% crop is: a part of an image that is viewed at the original resolution. Think of it like a picture puzzle, after you have it all together it makes a nice big picture, now if you take one of those little pieces out and look at it on it’s own, that is the same thing you are doing with a 100% crop. So this is an 800 x 600, 100% crop.
Source: Bill Huber blog.