Before the Jump
Jumping is jumping, right? Not much more to say about it? Well, if everything in a Street Fighter game was that straight forward, guides like this wouldn't need to exist.
Whenever you perform a Jump, it actually takes four frames for the majority of the cast to actually get off the floor. These four frames are typically referred to as "Pre-Jump Frames." If a character is hit during these four frames, they will actually remain grounded so that any ground combo will connect fully on them. However, during these Pre-Jump Frames, you cannot be Thrown. So if you start jumping the very frame something like a Spinning Piledriver would connect on you, you will escape and the Spinning Piledriver will whiff.
Also, the character performing the Jump is allowed to cancel these Pre-Jump frames with and only with Special Moves, Super Combos, and Ultra Combos. This gives you leeway when performing Quarter Circle Forward motions and accidentally hitting Forward Flip at the end of the Special Move code. You'll start to Jump, but if you hit the button early enough, the Special Move takes precedent over the Jump during the Pre-Jump Frames.
4-Frame Exceptions
Again, most every character has 4 Pre-Jump Frames. The only ones that do not are:
High Jumps
One character also have a High Jump (or Super Jump, as some call it): C. Viper. High Jumps actually have slightly modified behavior than regular Jumps. First off, you can actually be thrown out of the last 3 Pre-Jump Frames -- only the first Pre-Jump Frame cannot be Thrown.
Secondly, there is a small window right after the High Jump where no actions can be performed. When you High Jump and try to perform a Normal Move the instant you leave the ground, you'll notice you have to wait ever so slightly before a Normal Move, Special Move, Super Combo, Air Throw, etc. can be performed.
C. Viper, however, has one strange exception: the Pre-Jump Frames of her High Jump can actually be canceled by a Focus Attack. Even her regular Jump cannot have their Pre-Jump Frames canceled by a Focus, only her High Jump allows this.