B turnaround: you hit the opposite direction immediately before inputting the b move. With down b and up b you can just kinda angle to do this and with side B this doesn't work (nor is it really needed for the most part, since you can choose left or right already lol). The notable one is neutral b. So if you jump facing right, you can flick the joystick left (bringing it right back to neutral immediately) and then hit b.
B reversal (what you're talking about): you hit the opposite direction immediately AFTER inputting the b move. B reversal will also turn you around, but more notably, if you are in the air, the horizontal component of your velocity reverses, hence why it is useful to b reverse glide cancel. This can be done with any special, including side special, allowing for example Ike to side B left out of a run right (you can't normally do Side B's the opposite direction you're facing during a run... this is the only way). So you would hit right + B then immediately hit left.
Wavebounce: It has it's own name, but it is doing both a B turnaround and a B reversal on the same move. This is also semi-useful for glide cancel. The idea is to get the reverse momentum but to stay facing forwards. Generally I like people to be behind me when I'm above them as Zard, so I normally just B reverse, but it's not inconceivable that you would want to stay facing forward. For example, you could make like you are going to recover on stage but then wavebounce back and go for the ledge. I can't remember who, but someone was telling me it was a really strong tactic. I'm glad you reminded me... I'll have to try that out more, I don't think I've ever used it that way lol