I have studied improv in improv schools since 2003. The schools I have studied at are:

Comedy Sportz
Upright Citizens’ Brigade Theater (UCB) - (NY and L.A.)
People’s Improv Theater (P.I.T. -- writing only)
Magnet Theater
iO West

There are 2 basic styles of improv: short and long form. Comedy Sportz taught me short form, the others taught me long form. Of the 3 I have taken improv, here’s the BASIC breakdown of what I’ve heard each school is known for.

UCB: game
Magnet Theater: relationship
iO West (LA): take risks, scene study* (*editor's note 2/2/12 -- truth, I'm still trying to figure out what they're "known for")
Groundlings: characters (I haven’t studied there...this is what I ‘hear’ they’re known for)

Please remember, this what I’ve heard, that doesn’t necessarily mean that is what’s true. I have to say that those “titles” are sort of accurate to how the shows are performed, not to what is taught. I have taken classes at the same time at different schools and heard the same school of thought (UCB/Magnet/iO West/PIT coaches).

The structure of the levels in each school are different. I’ve picked up each of the schools’ summary so you can see it in one spot. The schools I picked are the one’s I’m most familiar with. There are many more schools out there in different locations, I just don’t know enough about them. If you want more details, please go to their respected websites:

Comedy Sportz (L.A. -- but this group is all over the country)
Short Form Improv Classes
Underclassmen: Beginner Improv -- basic skills
Juniors: Intermediate -- narrative driven improv with characters
Seniors: Advanced -- advanced narrative and characters
Long Form Improv Class -- occasionally offer these to expand your horizon

UCB (NY & LA)
101: Improv Basics -- fundamentals of long for improv
201: Game of the Scene -- identify and heighten the game, second beats
301: Harold Structure -- group games, learning the Harold form
401: Harold Workshop -- fine tuning the Harold, scenes, and games
Advanced Study: There seem to be different kinds of advanced study. Please refer to their website and see what is available.

Magnet Theater (NY)
Level 1: Improv Fundamentals -- principles of improv
Level 2: Improv Fundamentals -- intro to long form: scenes, group games, second beats
Level 3: Improv Core -- learning the Harold structure
Level 4: Improv Core -- learning advanced forms
Level 5: Improv Conservatory -- advanced training, learning advanced forms
Level 6: Improv Conservatory -- member of an improv team with 8 wk run

PIT (NY)
Level 0: a class for absolute beginners
Level 1: getting confident on stage, scene work, fundamentals
Level 2: scene work
Level 3: relationship-based scene work, long form technique
Level 4: long form technique, openings, group work
Level 5: scene work, develop own long form structure

iO West (L.A. -- iO Chicago: similar format -- please refer to website)
Level 1: Intro to Improv -- fundamentals of improv
Level 2: Character & Game -- building off of ideas, breaking down elements of game
Level 3: You -- find your comedic point of view and instincts
Level 4: Advanced Scene Work -- creating stand alone scenes
Level 5: Harold -- learning the Harold structure
Level 6: Advanced Harold -- getting creative with the Harold structure
Level 7: Graduate/Create Own Show -- create own improv form

Groundlings (L.A.)
Beginners:
Improv Workshop A -- fundamentals of improv
Improv Workshop B -- builds on fundamentals of improv
Intro to Sketch Writing -- learn different styles of sketches
Improv and Beyond -- connect the improvisor to the actor
Professional Actor
Level One: Basic -- fundamentals of improv
Level Two: Intermediate -- scene work and character development
Level Three: Writing Lab -- writing & performance of written material
Level Four: Advanced -- work with a group to put up a show

Second City (L.A. -- there is also one in Chicago and Toronto)
1: Improv Basics -- reviewing agreements of improv and building an ensemble energy
2: Improv Characters -- enhance improv skills and exploring characters
3: Improvising Scenes -- structure, content, character wants

I do not endorse any one school. I think I've gotten some kick-ass training from all the places I've studied at. Yeah, I know this sounds "politically correct," but it really is true. I'm pretty hard core about my research (as you can kind of see) and the right schools and teachers came to me at the right time. I do believe that UCB gave me my core training, which makes sense since this is where I started my long form training and this is where I've studied (and still study) improv the most. I've most recently been training with iO West and the experience I've had with my teachers (Shulie Cowan and Craig Cackowski) has been awesome. I will continue to train at UCB and iO West because I like and respect their teachers. I get what these people are saying and they give me the tools to get better as both an improvisor and actor (and also as human too).

If you're interested in learning improv. I suggest you go to the theater, see shows, see which performers you like, see what styles you like, and then talk to students and get feedback about the school and teachers. Listen to your gut. There's a lot of options out there and you really can't go wrong. Just keep in mind about the pacing and how quick/slow you're picking up the tools/forms of improv. Some schools might get into learning about "the game" too quick and some might be too slow for you. You'll also find that there are some teachers you'll learn better/easier than others...but that's another topic.