What is the difference between the "Chunk" and "Chunk Selector" input types?

While they are very similar, the biggest difference between a Chunk and Chunk Selector input is who decides what chunk will be inserted into the canvas. 

With the Chunk input type, the administrator creating the ContentBlocks field chooses the chunk beforehand. With the Chunk Selector input type, it is the editor who can choose from a list of chunks; however the administrator can still limit the available chunks through the field properties.