In total there are 6 visualisation options, which are based on 2 different ways to map the data, and 3 different visual styles.
So we end up with 3 'crawl' graphs: Crawl Map, Crawl Tree, Crawl Radial
And 3 'directory' graphs: Directory Map, Directory Tree and Directory Radial
The core difference betwen Crawl and Directory is that the Crawl graphs are mapping actual link relationships that were found by the crawler, whereas the Directory graphs are simply showing the hierarchy of the folder structure, and the connecting lines don't represent links.
View examples and explanations from the left hand menu.