Dumpster-diving in your city

Are their laws against it? Why do people dumpster-dive? Are they bargain hunters, needy people, flea market sellers, UPC rebate and refund people, or do they have other reasons? What are the best days for diving? What kinds of items can be found? Where are the best places? What are the consequences of getting caught -- or seen by friends? Grocery stores that throw away a whole carton of eggs if one egg is broken or throw away a whole bag of potatoes because one or two potatoes are rotting.