It's all part of the ongoing 50 state quarter program, started in 1999 (and originally proposed for the Susan B Anthony Dollars, but later changed to the George Washington Quarter Dollars, with the new Sacajawea dollars taking over for the SBAs). 2004 is the 6th year of the program, and will be quarters 26-30. Each of the three US Mints (Philadelphia, Denver, San Francisco) mints the coins, but the SF Mint only produces them for proof sets, and the Denver and Philadelphia coins go into circulation.
In 2009, provided no other states have been admitted to the United States, the reverse design on the Washington Quarter Dollar will revert to an American Eagle design (if another state is admitted between now and 2008, it gets its own quarter in 2009).
Other facts can be found at www.usmint.gov, or www.statequarters.com.