While we all talk about "Windows" as thought it was one big program, it is in fact made up of hundreds of small individual programs.
For Windows to work properly a number of these individual programs need to be running, and each one of these running programs is a process.
If you start a new "part" of Windows to do a particular job then another small program may start, and that would be another process.
I guess you are seeing this list in the Windows task manager, but make sure you are seeing them all. On Vista there is an option "Show processes from all usres" so you may have far more than 20 running.