Major League Soccer was founded in 1996 as yet another attempt to jump start professional soccer in the United States. In an effort to combat the overspending widely blamed for the collapse of the NASL, which lasted from 1967 to 1984, the league is organized in a unique owner-investor model in which all the clubs are technically owned by the league. It's a confusing setup but has so far worked to keep costs down and, after some initial stumbles, the league is in a period of solid growth. The San Jose Earthquakes rejoined the league in 2008, Seattle came on board for 2009, Philadelphia has joined for 2010, Vancouver and Portland for 2011 and two additional teams (not yet decided) for 2012. If you're doing the math that will be 20 teams by 2012, meaning the league will have doubled in size in only eight years.
Major League Soccer awards two trophies each season. The MLS Supporters' Shield (actually initiated by a group of MLS supporters groups) goes to the team that finishes first place in the regular season. Meanwhile the playoff winner receives the MLS Cup and is considered the league champion.
The Major League Soccer regular season runs from mid-March to mid-October and the playoffs from mid-October to mid-November.
Each team plays every other team twice (home and away) for a total of 30 regular season games.
3 points are awarded for a win, 1 point for a draw and 0 points for a loss.
At the end of the regular season the top two teams from each conference plus the next four in the overall standings qualify for the playoffs.
The eight teams that qualify for the playoffs are seeded based on regular season ranking, with the two conference champions getting the top two seeds.
The quarterfinal round is home and away, the semifinal round is single game (hosted by the higher seed) and the final is single game at a pre-selected site.
There is no relegation from Major League Soccer nor is there promotion from the second-tier USL-1. However the Seattle Sounders recently made the jump and Portland and Vancouver will be making a similar move in 2011 so the door is at least partially open for a well-run USL-1 team in the right market to eventually join MLS.
Details: 
now it's 8 teams in each conference. Each of the 16 teams plays all others once at home and once away - giving it a balanced schedule of 30 games.
the playoff qualification and format remains the same.