It is our intention that this tour provide enough description of the terrain to help you select the heading of the Mathematics Subject Classification appropriate for a specific inquiry. (There are 63 main headings and thousands of subheadings; the areas with index pages of their own offer a tour of their subareas and links to adjacent territory.) Click here to start the tour, or if you prefer, simply load all at once the shorter (41K) "Layman's Guide to the Mathematics Subject Areas", which contains most of the same comments but lacks the pretty pictures. Other types of navigation tools at the Mathematical Atlas: Steven Strogatz on the Elements of Math - Series. Online Mathematics Textbooks.
MathOverflow. Differential geometry reconstructed (free work-in-progress book)