Complex sentence: The NHL Winter Classic is one of the three series of regular season outdoor games played in the National Hockey League (NHL), and is distinct from the league's other two series, the NHL Heritage Classic and the NHL Stadium Series.
Split into simpler sentences: The NHL Winter Classic is one of the series of regular season outdoor games played in the National Hockey League (NHL). It is distinct from the NHL's other two series of outdoor games, the NHL Heritage Classic and the NHL Stadium Series.