High Side is when you get thrown from the bike, like over the handle bars for instance.
Low Side is when you go down with the bike on top of you and/or into a slide.
Low Side - the bike tips over, usually during a turn as a result of loss of traction. Basically it's like watching a bike fall over at a standstill, but you're going faster.
High Hide - The back wheel tragically catches up with the front wheel, and things just get worse from there. You're typically catapulted into god only knows what...
Of course I'm generalizing from a single example here, but everyone does that. At least I do.
Ok, here are the defintions right out of my MSF workbook...
LOW-SIDE CRASH
crash in which the rider contacts the ground behind the sliding motorcycle, usually as a result of front tire skid
HIGH-SIDE CRASH
crash in which the rider loses control and is THROWN to the ground in front of the motorcycle's path. This type of crash is likely to result in serious injury to the rider and is often the result of releasing the rear brake when a skidding rear tire is not aligned with the front.