According to Wikipedia:
In the NHL, each player signs what is, or is a variation of, a standard NHL player's contract. The contract specifies that the team has exclusive rights to the player playing in the NHL. Once an NHL player has played in a certain number of games or a set number of seasons has passed since the signing of his first NHL contract (see here), that player must be offered to all of the other NHL teams before he can be assigned to a minor league affiliate.[2] In the 2005 Collective Bargaining Agreement (CBA) assignment waivers applies only to loans from an NHL club to its American Hockey League (AHL) affiliate.[3]
After a player has been designated for assignment, the other 29 NHL teams can put in a claim or waive their claim for that player. The claims process starts at noon Eastern Time and ends 24 hours later. If only one team makes a claim for the player, then he will be transferred to the claiming team. If more than one team makes a claim, then the player will be transferred to the team having the lowest percentage of possible points in league standings at the time of the request for waivers. If waivers are requested outside of the playing season, or before November 1, then the player shall be transferred to the team with the lowest points in the preceding season.[4] If no team places a claim, the player can be sent to a minor league affiliate.
When a player clears waivers and is sent down and then is called up again, he does not have to clear waivers to be sent down again unless he has played ten games or has been "up" for 30 days.[4]