Don't confuse the silver eagle with the quarters in your pocket. The eagle is minted as a bullion coin, and was never intended to circulate. It is is a one ounce silver coin. Remember the kuggarnd from South Africa? Uncirculated verses proof refer to the way the coin is struck. There are some decent explanations already given.
As a bullion coin, you can buy them from the US Mint with the pretty packaging and pay the premium for that. The mint also sells them loose in bulk packaging to coin and bullion dealers. They are shipped in plastic tubes, unlike regular coins that are shipped in bulk bags, so there is little jostling during shipping. These dealers buy at a little above the spot price of silver, and sell them to us for a little bit more. They can be had for $18-26 each this way. Look for places like this:
http://www.apmex.com/?&promo=google|...FZJr7AodBCUAJQ
Look around this site for inexpensive holders:
http://www.jpscorner.com
You can get them alot cheaper, and find the years you want, by bypassing the mint.