Hooking in the rules
The NHL covers hooking in Rule 55, which defines it as "the act of using the stick in a manner that enables a player or goalkeeper to restrain an opponent." It goes on to specify that "when a player is checking another in such a way that there is only stick-to-stick contact, such action is not to be penalized as hooking." The NHL groups hooking with other "Restraining fouls" such as holding, interference and tripping.
Emphasis in NHL
Following the 2004–05 NHL lockout, the NHL made "Zero tolerance on Interference, Hooking and Holding/Obstruction" its top priority for game officials. Since that time the league and its officials have made a significant effort to follow up on that priority, and the game is being played in a more open style as a result.