You can only break combos 3 times per match.
So best not to waste them in the first round...
From what I have seen you can't break out of a juggle. Makes sense to me. The only time I would probably need to use a breaker is if I was being combo'd into a deathtrap, then I would want to get out.
~CYD

You can break out of air juggles aswell.

I think the way the breaker works is if somebody is doing a combo on you and if you press the same button they press at the same time that is the breaker it gets reversed seems very logic dont you think? And yes you can only break 3 times there is a little breaker table no top of the screen letting you know how many you have left.

All you do is tap >+Block.
You can do it while blocking or while being hit.
You can break out of a standing attack or combo.
You can also break out of a juggle the same way.
There is only one breaker animation and every character has the same breaker attack whether while standing or while being juggled. They just do a straight left punch.
The breaker punch causes 0% damage and always knocks the opponent down.
You can break out of "Combo style throws" with the same breaker. Throws that are like Scorpion's 2 punches and a knee.
You can break out of combo style throws late, for example one the 3rd hit of Scorpion's throw.
You are limited to 3. I'm not sure if it's per round or per match.
You can see many breakers being done in the Gamersday video. It's on this site. In that video they show the fighters breaking out of ground attacks and out of juggles.

It's 3 per match. If you use 1 in Round 1, you'll only have 2 left for Round 2 (and Round 3 if necessary)