The Waiver Wire shows players that are available to be claimed. You may claim a player if you don't already have that player on your roster (the same player from the same year cannot be on your roster twice) and you have enough available player units to be able to claim the player.
You are required to maintain a minimum roster size of 25 players (23 skaters and 2 goalies), but have no more than 35 players on your roster (this includes your minor league players). So if you are at the roster maximum, you must release an existing player to claim the new one. The player units freed up by releasing a player combined with any available player units you had after your draft equals your acquiring power - the maximum units you can spend to claim a player on waivers.
Players are claimed and released constantly as the same Waiver Wire is used by all leagues. So, if you release Brett Hull, he may be picked up by someone else in another league or by someone in your league and vice versa.
If you have more than the minimum of 25 players, you can release a player to free up some player units -- making your acquiring power greater.
Limitations: You must pay a 10% transaction fee for each player that you claim. For example, if you want to claim a 1 million unit player, you'll need to give up 1.1 million units. All Waiver Wire transactions must be completed by the transaction deadline - you will not be able to waive or claim players after that time.
Waiver Wire Unit Values (Full Season Leagues): You may notice that a player is priced differently on the Waiver Wire than he is in the Draft Center. This is because players on the Waiver Wire receive unit value reductions as they spend more time on the wire without being claimed. The Waiver Wire updates twice a day. With each update, all players on the wire receive a 2% unit deduction. So for each full day a player spends on the wire, his units are reduced by a total of 4%.
Target Numbers: The Waiver Wire updates twice a day. During each update, it makes sure it has exactly 25 centers, 25 left wingers, 25 right wingers, 50 defensemen, and 25 goalies. If there are more than the target number of players at any position, the players who have been on the wire longest will be cut until the target number is reached for that position.