When it comes to Window managers and Desktop environments, you will see why Linux is all about choice. There are so many of them, it's easy to lose track, and better still, you will be asked at installation time, which one you would lie to have. To make things worse, you can even choose multiple WMs/DEs, and decide which one to log into, when you put in your password at login time.
You might want to know a bit about them in advance, to be able to make an informed decision. the following few only represent the absolutely most popular and most widely used choices, for anything more exotic you will need to do your homework.
You might find out more about some of the major DEs under Discover Debian/Getting around the Desktop, these short articles are only aimed to serve as a first introduction to help you choose what to install.