What's the difference between a display manager and a window manager?

[Rick Thomas <rbthomas55@pobox.com>]

Showing my ignorance...  (I'm an old "command line" guy who mostly  
just needs a simple console terminal.)

Can anyone explain (or point me to a good document on) the following  
questions:



What's the difference between a "display manager" and a "window  
manager"?

Do I need both?  Can you give some examples of each and explain their  
features/advantages/disadvantages?

And how does any of this differ from plain old "X11"?

Thanks!

Rick