The window manager is the part of your linux system that configures X windows for you and gives you a better working enviroment.

You can run Linux as a command line operating system or with X windows.

Kde and Gnome put the cumbersome editing of configuration files firmly into the past - colors, fonts, menu bars and buttons can be changed using graphical configuration tools. Both desktops support Drag and Drop.

Can I use more than one manager?

Yes you can, when you boot linux at the login you'll have a list of windows managers. Depending on which you select you'll boot to this environment.

The K Desktop Environment (KDE) and GNOME provide a complete graphical environment under X. On top of a Window Manager, they also contain applications such as a file manager, editors, viewers and even some games. All these applications feature a consistent look and feel, i.e. all window elements such as buttons and menu bars look the same.

I suggest you check both their sites out, My personal choice was Kde as it is easy to use when coming from Windows