From Shoryuken Wiki, Marvel vs. Capcom 3 Strategy and More!
Remember what was just said? That every action in the game, including attacks, are "animated" frame by frame? Well, with regards to attacks, every attack in the game has three phases: a startup phase, an active phase, and a recovery phase.
You can see it in every move. Hit a button, and your character will start a move, hit the opponent, and then finish an animation. That's all these phases are: the period of time before your move hits the opponent, the period of time the move is hitting the opponent, and the period of time after the move hits the opponent.
So take those periods and break them down into those "animation frames" we just talked about. The start of your attack before it can hit the opponent is made up of Startup Frames. The animation frames during which your attack can actually hit the opponent are known as Active Frames. And everything that comes after those Active Frames are considered Recovery Frames.
These three types of frames make up the skeleton of every attack, and all three types are super important to various aspects of the systems and concepts that make up Super Street Fighter IV. So become familiar with these terms and keep them in mind throughout the guide.