Convolutional Neural Networks is a one-page (it’s a long page) introduction to convolutional networks written for CS231n, a course at Stanford. It somewhat shorter than the following resources (could be a pro or a con). It contains a nice animation halfway down the page showing how a convolution layer is applied across an image.
Convolutional Networks from the Deep Learning book, as usual for this resource, is fairly math-heavy and quite detailed. I recommend using it as a reference in which to look up specific topics for this week to gain a different perspective and possibly a few more details.