Morphological operations are very common in image processing. The two most basic of these are Erosion and Dilations, and from these, additional morphological operations can be performed. While there is an abundance of literature on the topic of morphology, I often like to view concrete examples of the operations.
So, I've published a few images that illustrate the output of 1-to-5 iterations of various morphological operations --erosion, dilation, opening, closing, gradient, top hat, and black hat--with various structuring elements--rectangle, cross, and ellipse.
The code to reproduce these images was written using OpenCV, and is freely available at https://gist.github.com/bradmontgomery/5096641