Currently (2013-04-01) the list of Leagues is:
- Bronze League: 8% of all players
- Silver League: 20% of all players
- Gold League: 32% of all players
- Platinum League: 20% of all players
- Diamond League: 18% of all players.
- Master League: The best 2% of all players. It is impossible to qualify directly into this League.
- Grand Master: The top 200 players in a region. Is created only after 2 weeks into a new season. Players that are not active for a certain time get demoted to Master.
Before you are placed in a League, you must play 5 Placement Matches against random opponents. After that, the game tracks your skill by assigning you a matchmaking rating (MMR), which is not exactly your League points, but similar. Different Leagues correspond to different MMR ranges, and it is possible to be promoted or demoted, if your MMR stabilizes within another League's range.
After a Season ends, all Leagues are wiped, but MMR does not. To be placed in a new Season, you have to play one Placement Match, which does not actually mean more than any other match - it is just there to filter inactive players from League rankings. Skipping several Seasons does reset your MMR.
Since some patch there is also a button to Leave League - if you feel that your MMR is well in another League (like if you are constantly encountering equal opponents from there), but a promotion just doesn't come, you can try to force it. Note that this doesn't reset your MMR, of course.
There is also a special, Practice League. When you start playing on a new account, you are placed here, and can play fifty games here before your placement matches into "real" Leagues (you can leave whenever you want though). Practive League matches are played on a special, more "noob-friendly" maps, that have destructible obstacles completely blocking bases off, eliminating early pressure. After you leave Practice League, there is no way back ever.
Further reading: Starcraft Wikia
No, you are not stuck. As you keep playing, you can move between leagues depending on how well you do.
I recommend leaving the practice league as soon as you are ready to start "getting better", since the "real" league gameplay is quite a bit different. First, the game speed is faster. Second, the maps don't have walled-off entrances to protect you from rushes. I'm not sure that spending a lot of time in the practice league will be the best help when it comes to placement matches.
Best Answer
The practice league is usefull if you never played SC2 multiplayer. It uses
Novice mapswhere the entrances to both main bases are blocked by destructible rocks which prevent you from being rushed and shuts down many cheesy builds like cannon rush. Through this you have some more time to build up and get used to your buildings and units until you engage with your opponent.Although when you get out of the practice league you maybe will have to readjust your play since there no longer is any early game protection.
IMO its better go do some custom games vs AI or people you know to get used to multiplayer and the maps.