Sports term for the main
schedule of games for a
league. The regular season follows a shorter
preseason (during which
statistics and
results do not count) and precedes the
postseason (or
playoffs) which culminate in the crowning of a
champion.
Regular seasons serve many purposes. They're used for positioning for the postseason. For example, in the NBA and NHL, 8 teams from each conference qualify for the postseason. Therefore, teams need to finish that high in order to take part in the playoffs. Additionally, finishing especially high would help a team to get home-field advantage for the postseason, thus helping their chances for a championship.
Additionally, they're used for statistics. Just about any famous sports records (Joe DiMaggio and Cal Ripken's streaks; Roger Maris, Mark McGwire, and Barry Bonds' home run records, Wayne Gretzky's points mark, etc etc etc) are from the regular season. From a historical perspective this is a big part of what makes sports so popular through the years. The ability to compare players and teams' performances is largely due to regular season statistics.
Regular seasons are usually very long compared to the postseason. In major league baseball, the regular season is 162 games for each team (as compared to several rounds of best-of-7 playoff series in the postseason). Similarly, NBA and NHL regular seasons last 82 games each. The NFL regular season has each team playing 16 games, while the maximum postseason games a team can play is only 4.
It should be noted that most soccer leagues worldwide go not have a postseason, so the league champion is crowned based on regular season standings.
Just like one eats a large meal and often gets to a smaller fabulous dessert that they'll remember long after the meal, sports regular seasons make up the bulk of the sports calendar, before the playoffs crown a champion. And sometimes they're as memorable as the playoffs that follow.