Blocking is a ubiquitious and highly important element of Street Fighter battles. A character may perform either a standing block, or a crouching block. To perform a standing block, press away from your opponent on the lever. To perform a crouching block, press away and down from your opponent on the lever. Your character will *only* block if the hit box from an attack is near you. So if a character is backing away, throwing jabs from the opposite side of the screen will not force them into a blockstate. Standing blocks will deflect high, middle, jumping and overhead attacks, while a crouching block will block middle and low attacks, such as sweep kicks.
Once an attack is actually blocked, the character enters a blockstun state. This means that the character, until the blockstun passes, is unable to perform any actions besides switching from a standing block to a crouching block. If you are hit again during the blockstun state, you are forced into it again, meaning that until the attacker finishes their combo, you have no choice but to continue blocking it. (On a plus note, at this point the character is also immune to throws, and will automatically stay in a standing block state until the blockstun passes, although you still have to specify a crouching block.)
If the blocked attack is a normal attack, your character will take no damage, although in some incarnations of Street Fighter they may take guard damage. (Usually guard damage is represented by a meter below your life bar.) Once your guard damage meter runs out, you suffer from a guard crush. When your guard is crushed, you are stunned momentarily, and unable to defend any attacks. Once the guard crush passes, though, your guard damage meter will reset. Also, if you go for a period of time without blocking any attacks, the guard damage meter will refill.
If your character blocks a special or a super, on the other hand, your character will lose a very small amount of their life bar for each hit of the special or super that is blocked. So if Ryu opens up with a Shinkuu Tatsumaki Senpuukyaku (Super Hurricane Kick), and you block it, you'll still take a bit of damage from all the blocked hits, as well as guard damage.
It is impossible to block a throw.
Basically: The ability to block while in the air.
Air guard only works against certain attacks, though. The best way to realize which ones will work and which ones won't is that if a character hits you with an attack while they are still on the ground, air guard will not defend against it. So if Sagat hits you with a standing h.kick while you are jumping toward him, you will not be able to block it, although you would be able to block a jumping m.kick.
As for the shoryuken style moves, only the first few execution frames of the attack are registered as being on the ground. So if you time a shoryuken correctly against an air-guarding character, they will eat the uppercut.
There are a few moves which *look* like air moves, but still aren't defendable by air guard. These include moves like Terry's standing h.kick, and Mai's standing m.kick.
Air guard also does not work against supers.
A "parry" is a special, last-second defensive maneuver that allows a character several advantages over a regular block. This system was first implemented in Street Fighter 3.
Firstly, a successful parry will slightly charge your characters super meter. In Capcom vs. SNK 2, the meter will be charged by 2.6%. (5 points out of 192).
Secondly, a successful parry carries no blockstun frames, and eliminates knockback, allowing you to punish your opponent with a special, a combo, or even a well-placed super immediately after parrying the blow, while they are still in their recovery frames. Even better, after a successful parry, the attacking player will be frozen for a split second, affording you extra time to punish them. This advantage is powerful enough that a proficient player using parries will dominate over just about any other defensive options.
Parries also negate any block damage.
Keep in mind that parries do not work against throws of any sort. They also cannot be executed from a blockstun state in CvS2, although they could in Street Fighter 3: Third Strike. Furthermore, parries cannot be used during a small jump.
The player must decide right before the attack connects whether or not a low parry or a high parry is required, depending on the height of the attack. If the wrong parry is selected, it is whiffed, and the character will be hit by the attack. A good rule of thumb is that if you must crouch to block the attack, then you need to use a low parry. A parry is executed by pressing forward for high attacks, down for low attacks, and forward for all air attacks. You cannot perform a parry in the air if you've performed an attack during that jump. Parrying input lasts for 8 frames on the ground, and 7 frames in the air if the lever is reset to neutral within 3 frames of execution. Otherwise, the parry input only lasts for 4 frames. The risk factor here is that in order to get the mechanical advantages of a parry, the character must briefly place themselves in a non-defensive position (by pressing forward), while at the same time guessing whether the attack will be high or low, meaning that if the parry is whiffed, they will be hit.
Parrying does work against supers, both in the air and on the ground, although each consecutive hit must be parried on it's own.
Another disadvantage of the parrying system is that your character is not considered to be in any reel stun or blockstun state, as covered above, but this also means that they are vulnerable to throws. This means that if you parry an attack that is buffered into a fast command grab or grab super, you will be susceptible to these attacks.
"Just Defend" is just what it says. It is a system which rewards you for "just defending" an attack, which is accomplished by pressing back right before the attack connects. You get several bonuses for just defending an attack.
First, your character receives a small amount of their life bar back. This can be extremely frustrating for an opponent when they are pummeling you with fireballs from across the screen, and you just use them to refill your life bar.
Second, it will charge your super meter.
Third, you do not get pushed back like a normal block, you take no block damage, and the blockstun state is far shorter than it would normally be.
Just defending is generally safer than parrying, because you aren't having to push towards the attack, so if you miss the just defend, you'll probably still block the attack. However, it doesn't open the window for instant retaliation the way that parrying does.
You cannot, of course, just defend a throw.
You can just defend in the air, allowing your character some form of defense during a jump. You cannot, however, perform a just defend if you have attacked during that particular jump.
When you press back on the joystick, any move that hits within 6 frames will be just defended. This translates to 1/5 of a second in real time. However, if you return the level to neutral, the just defend is lost, so it's best to hold the lever back even if it means missing the just defend. You cannot just defend from a blockstun state, unless the blockstun state originated from a just defend. You also cannot just defend during a small jump.
To perform multiple just defends in a row, the lever needs to be reset to neutral within 6 frames of the last just defend.
A dodge is very similiar in mechanics to a roll, with the noticable difference that you do not move as you do during a roll. This system originated from the Extra Mode from King of Fighters and was implemented into the Capcom vs. SNK series. A dodge is executed by pressing LP+LK.
There are three stages to a dodge, but all three of them afford invincibility to all attacks. The first stage shows the character leaning back, the second shows them in a (sometimes) very cool looking pose, and the third is them leaning back into the battle. The best thing about a dodge, though, is that if you hit an attack button while you are in the second stage of animation, your character will launch a predetermined attack directly from the dodge. (This is similiar to, but not technically, a reversal.)
For each character, there are two possible attacks from a dodge. The first is always long-range, and will knock down your opponent. The second is always short-range, and bufferable, even if the attack it mimics isn't normally bufferable. So if you dodge an attack, and come out with bufferable attack, feel absolutely free to buffer it straight into a super. 0wnage.
Even better, dodges have no vulnerable phases. Even if an opponent's attack is still extended into your sprite when you come out of the dodge, just hold back, and you will block it. A dodge is also bufferable... INTO ANOTHER DODGE! This means that if an opponent is trying to use a long-lasting super to kill you with block damage, you can just just chain dodges together to avoid the entire thing. And to add feature onto feature here, you can even use a dodge as a reversal, going into invulnerable frames directly from the ground. Then you can cancel the dodge into an attack, buffered into a super... Well. You get the idea.
Just like rolls, dodges are vulnerable to throws. You can't have everything, I suppose...