Roll cancelling is actually a glitch in the Capcom vs. SNK 2
engine, but it is a glitch that has become a very important part of tournament play.
We'll start with an example of a roll cancel.
Let's suppose you're playing Ryu, battling a particularly nasty little player who prefers Blanka. You're keeping him from chewing your head off by pounding him into the corner with fireballs, patiently biding your time while burning off the clock. Suddenly, you see Blanka go into a rolling attack! You grin to yourself as you see your fireball between yourself and Blanka, then the grin freezes into a rictus of disbelief as Blanka goes right through your fireball, and reams you with his rolling attack. The momentum has shifted now, and before long, the match is over.
So what the hell just happened?
This neat little glitch is a result of what the Japanese call "kara cancelling". (Kara is a Japanese word meaning "empty".) We're all familiar with buffering, right? In short, it is cancelling a normal attack into a special. Kara cancelling is basically the same as buffering, except it's buffered off of a missed attack. This was implemented by Capcom in the very first Street Fighter games to allow leniency in performing special moves.
Roll cancelling is basically the ability to kara cancel a roll into a special attack. This changes the roll *animation* into that of a special attack, but the engine does not reevaluate the invincibility frames from the roll. This means that for the first 21-23 frames of your special (about 1/3rd of a second), you are completely invincible.
The implications of this are huge. You can have 100% invincible anti-air attacks. You can attack right through an opposing characters super. As outlined above, you can attack an opponent right through a fireball. You can even roll cancel your fireball to go right through your enemies fireball!
Fortunately, despite how useful roll cancelling can be, it is wickedly difficult to do reliably. You can only cancel a roll in the first three frames, which, in case you don't know, is equal to one tenth of a second. And messing up a roll cancel can lead to, needless to say, very undesirable results as you will usually perform the special move, but with no invincibility frames.
It also should be noted that you cannot roll cancel into a super. Well, actually, you can, but since a super has its own invincibility frames, it causes them to be reevaluated, so there isn't any real advantage to doing so.
For extra style points, try roll cancelling into a taunt.