This here:
http://www.w3schools.com/js/js_cookies.asp is everything you need to know about cookies from a programming prospective(Javascript in this instance, but PHP/whatever could also be used)...
On a basic level:
- A webpage stores a cookie on your pc in order to save information about you/your actions on the site
- The webpage checks to see if theres a cookie stored on your PC when you open the page.
- The webpage acts accordingly.
The W3Schools link above goes into detail on the whys, hows, etcs.
Why do you want to know about cookies anyway?
Cheers,
Jamey