Clutch Questions?

Hey everyone. Wondering if anyone can help me out with some clutch questions:

1) How do you know if your clutch is worn out?
2) What does it mean when your clutch is slipping?
3) How do you wear out your clutch (ie. if I keep my foot on it, fully depressed, will that wear it out. Actually, is that a bad idea, as long as I don't have it partially disengaged)?
4) How long does a clutch usually last for (# of years or miles)?

Any information is greatly appreciated. Thanks.