Teams use different systems. A 1-2-2 sends one player in deep while two support. A 2-1-2 sends two in aggressively. Aggressive forechecking teams are hard to play against because they never let you make a clean breakout.
Forecheck
Pressuring the other team in their defensive zone to force turnovers.
More in tactics
Own goal
Scoring on own team's net
Odd-man rush
Attack with more attackers than defenders
Backcheck
Skating hard back toward your own end to help defend. Backchecking is one of the most important habits in hockey. The best players in the world backcheck. When a player does not backcheck, it creates an odd-man rush, which is one of the highest percentage scoring situations. Players who refuse to backcheck will find themselves on the bench.
Neutral zone trap
Defensive system clogging middle ice