Well every time someone falls down on the floor it's considered a knockdown lol. So a simple sweep or throw (or any other move that makes the opponent fall down) gives you a "knockdown".
When you get knocked down you can stay on the ground (by holding down) or roll back (by pressing back). And when you're about to stand up you can perform a wake up move. Basically any special move done as you get up is a wake up move. They often have some type of invincibility but not all of them. That's the "wake up" phase. So when you hear people say that a character has a poor wake up game, it often means they have bad reversals or options to get away from pressure after they get knockdown.
Some knockdowns are "untechable", meaning you can't roll out of them. Like Sub-Zero's slide for example, that gives you an untechable knockdown.
