Quite Stunning
The concept of Stun, or "falling dizzy" as it is commonly referred to as, has existed since Street Fighter II Classic. Basically, if your character is struck too many times in a row, your character will fall dizzy. As soon as this occurs, even in the middle of a combo or if the move that knocks you dizzy is just a Normal Move, your character be knocked over and fall down onto the ground. Stars will begin rotating around your head the instant you are knocked dizzy, so you will see the stars while they are falling over. When you get up, your character will be in Stun, and you cannot perform ANY actions. Your character is thus susceptible to anything the opponent chooses. To put it more bluntly, the instant you become Stunned, the enemy gets a free combo.
Once your character falls dizzy, he/she won't stay dizzy forever. After a certain amount of time your character will recover and return back to a normal condition, but it takes a while. While you are dizzy, however, you can try to come out of dizzy faster by shaking your controller and mashing on the buttons as fast as you can. Doing this will make your character recover quicker than normal, but be careful not to go overboard with the shaking... If your character does shake out and you are still wiggling the joystick, you may eat the combo your opponent is going for anyhow and end up taking more damage than if they had hit you while you were still dizzy. So shake as much as possible, but right before your enemy strikes you, just hold Block. If you shook out, you'll Block their attack and be safe. If you didn't shake out, wiggling the controller right before the enemy hits you won't make a difference anyhow.
The Stun Meter
The Stun Meter cannot be seen anywhere on the screen during a normal match, but is kept track of "internally". The only place you can see it is in Training Mode. The Stun Meter is just a counter with a max value, and every time you get struck, the counter increases. This is referred to as taking "Stun Damage". And once the counter reaches the max, your character will fall dizzy.
Every move in this game gives different amounts of Stun Damage, and there's no formula to it. It's pretty much an arbitrary value assigned by the developers of the game.
Almost every Super Combo and Ultra Combo in the game do not cause any Stun Damage. In past games, where they did, characters falling dizzy in the middle of Supers was always more of a nuisance than anything. So in later games, Supers (and now Ultras) cause no Stun. There are the couple of exceptions, of course, such as Balrog's Dirty Bull and Gouken's Denjin Hadoken. And in the case of the Denjin Hadoken, you even gain Stun Damage while blocking the move!! However, even if your Stun Meter fills all the way up while blocking, you won't fall dizzy until the opponent hits you one more time.
The Stun Meter does start to go down after a fixed amount of time. Basically, if your character is not hit for a about one second on the game clock, your character's Stun Meter will start draining back down to "zero". But anytime an opponent connects a move on you, whether it hits or is blocked, the Stun Meter will stop draining and you'll have to wait that same fixed amount of time before it starts to drain again.
Thus, you can be 100 points away from being Stunned, and continually block attacks for 20 seconds on the game clock, and then get hit by one more move and still be Stunned. Since you never avoided attacks for that fixed amount of time, your Stun Meter never had a chance to start draining. Thus, if you know you're close to being Stunned, do everything you can to avoid getting hit to prevent yourself from falling Dizzy within the next few hits.
Also, that one second delay before the Stun Meter starts draining is actually one second after you can perform your first action. In other words, if a move knocks you down and you lie on the ground for more than a second, your Stun Meter doesn't start draining while you lie there. The game will wait until you get up before the one second count begins.
Getting Dizzy
As soon as you are knocked dizzy, your Stun Meter drains to zero. Then, any further hit in the combo (falling dizzy does not reset the Combometer) will not add any Stun Damage. So after the enemy finishes comboing you from a Stun, you will start up with zero Stun Damage. Also, when you recover from a dizzy, your Stun Meter increases it's max by a 20% value. For example, Dhalsim, who has 900 Stun Points, will go up 180 points to 1080 Stun Points if he recovers from a stun. So after getting dizzy once, it becomes harder to get dizzy again. However, the next Round, things reset to their defaults, so the fact that your Stun Meter increases rarely makes any impact on the game at all since it's very difficult to get stunned twice in one Round.
== Stun Meter Lengths
Again, every character has a different sized Stun Meter, just like they different sized Life Meters. Again, 1000 Stun Points is the average for the characters. If you have 1100 Stun Points, you are hard to knock dizzy. If you have 900 Stun Points, you are more easily stunned.
Here are the Stun Point counts for every character in the game: