MLS is an American league that doesn't use a promotion and relegation system like other leagues around the world (but, Australia/New Zealand). They also use a split conference (east and west) that is popular with American soccer fans, but there are people that do wish they would use a single conference since you will see only one cross conference match. Although it's an 18 year old league, people get on it for not being as perfect as a 100 year old European league. When you watch MLS, you will have to understand that it is still in it's development stages. MLS is still adding new teams and don't know a clear limit to when they should stop. I heard 24 for a long time and it's been upped to 26.
The person that is in charge of MLS is Don Garber. He is a OK commissioner, but not the one MLS need to succeed in the future. He has been involved with other sport groups before joining MLS, but he shows that he has no clue how to run it. MLS need someone to run it that knows the sport. It would help if that person played and coached in a soccer club before trying to run a league.
MLS is not just an American sport. Three Canadian teams also play in this league (Toronto, Vancouver and Montreal). MLS has three trophies you can win, MLS Cup (end of the season tournament against the top five teams in each conferences), US Open Cup (like Briton's FA Cup, it's a tournament against all the division, but Canadian teams aren't allowed to participate) and East/West regular season champions with Canada get a separate tournament championship.
The winner of the MLS Cup, US Open Cup, Canadian Cup, Eastern Conference and Western Conference get to be in the CONCACAF Champions League the following year. The CONCACAF Champions League is a North and Central American tournament where the winning get to be in the FIFA Club World Cup. A MLS team has never won it since there was a Club World Cup spot up for grabs, but one team (Real Salt Lakes) did make it to the finals once. Last year, two MLS teams made it to the semifinals of the tournament, but lost to Mexican teams.
Note: There are no tournaments against South American teams like there used to. MLS used to compete in tournaments against South American leagues, but it ended for no reason. MLS actually won more than once against South American and Mexican teams, but only a small few teams where winning it. When you get into MLS, you will hear rumor about CONCACAF (North, Centrol and Pacific Island America) and CONMEBOL merging once in a blue moon because our confederation is full of weak teams while CONMBOL is known as a small confederation.
Don't expect to see your team every week on TV if you don't support your local team.