Rookie League

This is were most players are placed after they are drafted.  It is the lowest level.  Most guys spend half a season here.

Single A

There are two levels of A ball (low A and high A).  Between these levels you would probably spend two years....if your good enough to get past low A.  This is as far as most guys make it.

AA

More today then ever AA is the most important level.  This is were the biggest adjustment in competition is.  Normally guys will spend one and a half years here if they don't get cut before that

AAA

Today triple A is more of a level for old guys that can't perform at the major league level anymore.  Although many prospect will still spend less than a half a year here.