The Waiver Wire can be found by clicking on GM's Office. 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 can not be on your roster twice) and you have enough available cash to afford to claim the player.
You are required to maintain a full roster of players so in order to claim someone, you must release an existing player first. The salary freed up by releasing a player combined with any available cash you had after your draft equals your buying power - the maximum 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 Dan Marino, he may be picked up by someone else in another league or by someone in your league and vice versa.
Limitations: You must pay a 10% transaction charge for each player that you claim. For example, if you want to claim a 1 million player, you'll need to give up 1.1 million. All Waiver Wire transactions must be completed by the end of the regular season, i.e. you will not be able to waive or claim players after that time.