FroggieStyle asked for a brief analysis of the levels, so here you go. (In reverse order)
Extended Spring Training
- Where you go to work out if you have an injury, or if you are refining something, or if you are 17 years old from the Dominican. It is a faux league where the rules are sometimes twisted so that teams can get guys into the proper developmental situations. Eventually, this turns into the Grapefruit Summer League or Cactus Summer League, depending on what state you are in (Florida, Arizona). This is a very young league.
Rookie Summer League
- This is the summer league that takes place at the teams spring training complexes, and is typically a first stop for drafted players (college or high school) after they sign, Dominican/International signs, or guys working out of an injury or refining command/control/approach. A very young league.
Short-Season A Ball
- This is a summer league for mostly college players after they are drafted, and some journeymen minor leaguers who need to work on something at a higher level of competition. Some high schoolers and international signees show up here if they are at an "advanced level of development", but it is generally an older league.
- This is the full-season A-ball league, where you find mostly washed up journeymen, or super young prospects with bright futures. Guys come and go from this level frequently, typically starting here after minor-league spring training, and then either leveling up at some point or getting their release after the draft (new guys take their spots). It is a good mix of young and old.
- This is a more advanced full season league, similar to Low-A ball but also more challenging. If you are in high-A ball, your club likely thinks you could be a serious prospect and they just want to see you continue to refine and get better before promoting you to the next level. This is a mixed age league, but is pretty old. Getting to high-A is hard, but the jump to the next stop is the hardest jump in all of minor league baseball.
- This is the developmental mountain top of the minor leagues. If a team promotes you to AA, they believe in you and want to see you succeed. This level is much older than those below it, lots of vets and advanced level prospects. You will see a lot of hard throwers in the lower levels, but in AA you see real pitchers developing. From AA, guys can go to AAA but major prospects often jump up to the Majors from here. If you prove you can play here, most teams believe your next best developmental stage is in the bigs where you get the best exposure and coaching possible. Some guys come here and fail, and are sent down or released if they can't hack it.
- The holding ground of broken dreams... Mostly made up of "AAAA" players who kill it in the minors but could never break through in the bigs... Some prospects are here too if they need more refinement, or if they are blocked at the major league level. The competition is higher than in AA, but not so much so that it makes a tremendous difference. If you play 20 years of AAA ball, you can make a small living for yourself, but it isn't much of one compared to the living in MLB.
- If you are coming out of HS and sign after being drafted, you will likely go to rookie ball for the summer. If you are a high level prospect or if you are killing it in the league, you may get sent to short season A-ball for a month or so. The next spring you will likely start in ext. spring training, then go to low-A. Some teams, like the Rangers, like to get aggressive with high upside prospects with good makeup, and they often send them to high-A at this point. But, regardless, it isn't uncommon for a prospect to stay in high-A for a season or two. If you make it to AA, you are in the game, because success at this level is an indicator of potential ML ability.
- If you are coming out of college, you will likely still go to rookie ball for a month or so after signing, before going to either short season-A ball, or maybe high-A to finish out the year. If you are Stephan Strasburg, you could go to AA, but that is pretty unlikely. The following year, you will likely go to high-A to start the year (or low-A, depending on your ability).
The 40-Man Trumps All
- The 40 man roster is a roster major league teams keep of guys in their system they would like to be able to call up to the majors throughout a season. Getting on the 40 man is a big deal, your pay goes up and their is a certain level of security associated with it because to get you off the 40 man roster, the ML team must put you through waivers, meaning any ML team could claim you if they wanted to. If you aren't claimed, you can be sent down to a minor league team, or released. The ML roster is made up of 25 men during the season until September, all 25 of those players are on the 40 man roster. The rest of the 40 man is likely in AAA or AA, with the occasional player in high-A (like Matt Purke). In September, ML rosters expand above 25 so that ML teams can call up players from the minors to be with the ML team, part to rest the regulars more in the home stretch and part to help get major prospects acclimated to pro-ball life and the environment of a big league team.
If you have any more questions, through them in the comments section!