As defined by Wikipedia...
"HTTP Cookies, sometimes known as web cookies or just cookies, are parcels of text sent by a server to a web browser and then sent back unchanged by the browser each time it accesses that server. HTTP cookies are used for authenticating, tracking, and maintaining specific information about users, such as site preferences or the contents of their electronic shopping carts. The term "cookie" is derived from "magic cookie," a well-known concept in UNIX computing which inspired both the idea and the name of HTTP cookies."In essence, cookies are used by sites you visit to see what has changed since your last visit. I personally don't like them and have them turned off until I feel that I can trust the site. This site is trusted and therefore, I keep cookies turned on. There are just a few sites that I trust. Otherwise, my cookies are turn off all the time.