What's revision control?

This is the css directory for one of my clients, just for one file:

 ++main.css
 __main.css
 main-17-october.css
 main-1st-nov.css
 main-23rdfeb.css
 main-3-8-9.css
 main-bk1.css
 main-extra.css
 main.css
 main.css.bk
 main14April2009.css
 main15April2009.css
 main_bk.css
 main_old.css
 main.css.bak

Not only are their 14 versions of that file, but their are 5 separate conventions for assigning dates. Truly remarkable.