A new guy question, from me.
 

The plastic piece on the rear wheel, behind the cassette, what is the purpose of that? I honestly don't know.

If you ever accidentally overshift the cassette, your chain will bugger up the spokes on the outside of the hub flange faster than it has taken me to type this. That plastic disc is a spoke protector and is supposed to keep that from happening.

Also called a dork disc. But it is to protect the spokes in case of a maladjusted derailleur or an over shift.

Aaron :)

well, I learned something. Thanks!

Was riding a dumpster-found mountain-bike-shaped-object yesterday, one that I had already serviced/cleaned/inspected/test ridden (hard, I might add), and heard a strange grinding coming from my rear derailleur. I looked down and saw that the chain seemed to have jammed between the plate of the derailleur cage and jockey pulley... well, me being with a friend at the time and only about two blocks from my house, I kept pedalling hoping it would jump back on. After a few strokes I heard the unmistakable sound of something grinding against spokes, yelled out and expletive to my partner and coasted to a stop at the bottom of the hill. The jockey pulley was missing, the plates misaligned, and the chain had managed to jump from the middle of the cassette to the inside, wedging itself between the "pie plate" and the largest cog. Seems the derailleur cage was what was smacking against my spokes, but the spoke guard managed to keep the chain from tangling in the rear wheel.

If I cared about keeping the bike, the pie plate saved a lot of potential damage to the spokes. Then again, if I cared about keeping the bike, I would not have picked one up with a Tourney derailleur. At least whoever at the local bike co-op gets the un-enviable job of replacing the RD won't have to worry about replacing spokes (given how often I'm there it'll probably be me in any case).

It's a Fred indicator and indicates to other riders this rider is a Fred. If you don't know what a Fred is, you need to HTFU.

A couple of months ago I got tangled up with another rider and we went down. Unbeknownst to be, my RD hanger got bent. A bit later the bent hanger caused me to overshift and drop the chain past the large cog into the spokes of the rear wheel, one of which subsequently broke. The hanger is now straightened and all the drive side spokes replaced. A dork disk would have prevented the spokes from getting damaged. Unfortunately I'm not a Fred so can't run one, otherwise I would.

I hate breaking 25% of my spokes all at once like that.