Source: www.sweden-poker-online.com

Rebuy tournaments permit players the option to buy more chips during a limited time period which is referred to as rebuying.

Many Poker players love rebuy tournaments because of the low initial buy in with the option to increase one's stake if the player feels confident at the table. On the other hand these tournaments turn off many players because the better heeled player can sometimes outlast the rest of the table with necessarily being the best player.

"Poker News" has put together a great article on the pros and cons of rebuy tournaments.

And here's a good explanation of the pluses and minuses of freezeouts vs rebuys plus links to other Poker basics.