Early in 2011, the U.S. Mint announced they would start to mint the bullion 2011 Silver Eagle at the San Francisco Mint, since they could not longer produce the quantity needed at the West Point Mint.
As a result, PCGS, NGC and ANACS said they would grade the Silver Eagles with a -(S) or -(W), providing the coins were submitted in sealed monster boxes, which had the bands around them saying which mint the coins were produced in.
Fast forward to October 2011. The U.S. Mint produced the 25th Anniversary Set of five Silver Eagles, and included a 2011-S Silver Eagle. Because the "S" is not in parenthesis, it means the "S" is on the coin. This coin was only available in the 25th Anniversary Sets and its mintage was limited to 100,000.
Please be careful, when searching online for 2011-S coins that you are actually bidding or buying a "S" mint coin, since these sell for more money than the 2011-(S) from the bullion monster boxes.
Both coins are very popular though!