What is the meaning of "without contract"?

What does it mean if a cell phone is without contract? Does it mean it is unlocked (able to use any sim card from any provider), or just that you have to use the same company's plans? An example is on ebay at the given link. Does that mean that that phone can be used with any cell phone provider or just that particular provider? THANKS A BUNCH!! :) http://cgi.ebay.com/T-MOBILE-DASH-s620-HTC-UNLOCKED-PDA-GSM-WINDOWS-PDA-PHO_W0QQitemZ170261151747QQihZ007QQcategoryZ3312QQssPageNameZWDVWQQrdZ1QQcmdZViewItem