A way to keep your sanity while at work. Most people can't stare at the same problem for 8 hours a day. The solution is to have something else to do when your brain fries.
Some people play games (like Solitaire or Minesweeper), others surf the web, while the intellegent people spend their breaks noding. Tired people will take catnaps on breaks. There's also bathroom breaks.
Length and occurence of breaks vary widely. Some have their break schedule dictated to them by their employer. Others take them when ever they want. Some breaks last only a moment, but others may last for an hour or more. Lunch break!
Of course there are those that just goof off at work. For them, breaks are actually doing what they're supposed to be doing.