Anti-Airs
Anti-airs are just another form of reacting to something your opponent has done, though it can get more in-depth, so I think it deserves its own section.
When you read the words Anti-airs, what comes to mind? For me what comes to mind is keeping something out the air, exactly what the conjunction of the two words mean.. Put simply, anti-airing your opponent is the act of keeping them from jumping. Since jumping in is one of the best ways to start a combo in Street fighter, anti-airing is the best way to eliminate that aspect of some ones game. The way it works is by you pressing your grounded move which has more priority over your opponents in-air jumping move, essentially beating it. Check out the video if your still alittle lost.
Of course every character has an anti-air of their own and some are better than others, it's your job as a player to figure out your own characters strengths and weaknesses. Practicing with anti-airs is a good way to start that process. In order to do this you need to also understand how the mechanics of the game works as well.
The game is set up in terms of hit boxes. Each character has a hit box relative to their size, and the shape of that hit box changes with movement. Anti-airing is no different, it still follows all the same rules put in by hit boxes. In order to land a hit in SSF4, your characters hit box has to collide or make contact with the opponents hit box, if they don't neither character will land a hit. I'll give you an example of this.
In the image, the hit boxes are shown as green/red images. Green represents the physical body of the character and all areas that can be hit and Red represents the area your character can hit another while still maintaining their own area of effect. If the two hit boxes doesn't connect, neither will take damage, but if they both hit at the same time, both characters will take damage.
Understanding these mechanics is vital to advancing in the game.