In theory, offer sheets provide possibly the most exciting mechanism the NHL offers for player movement.
Teams can offer contracts to any restricted free agent around the league. If the player signs the offer sheet, then the their team has seven days to either match the contract or concede the player to the other team, receiving a pre-determined allotment of draft-pick compensation in return. It's an opportunity for teams to poach quality players who are in their early- to mid-20s.
In practice, offer sheets are a rarity. Since the salary-cap era began in 2005, only nine have been signed, including just one in the past seven offseasons.