Many people think that a ball that bounces over an outfield wall and out of play is a ground rule double. Actually, this is not correct, though the usage has become so established at this point that it's not likely to ever be corrected.
Major League Baseball Rule 6.09 (e) states that "A fair ball, after touching the ground, bounds into the stands, or passes through, over or under a fence, or through or under a scoreboard, or through or under shrubbery, or vines on the fence, in which case the batter and the runners shall be entitled to advance two bases...." This is specified in the rules, and is a very different thing from ground rules, which are rules specific to the park in which a game is being played.
For example, in Fenway Park there is a ground rule that says that any ball that goes through the scoreboard on the Green Monster on the fly or on the bounce entitles the batter to two bases. In Wrigley Field, a ball becoming stuck in the ivy on the wall in front of the bleachers is two bases. These rules are specific to the grounds (park) in which the game is played, and are examples of true ground rule doubles.
A complete list of ground rules for Major League parks can be found at
http://www.mlb.com/NASApp/mlb/mlb/official_info/mlb_umpires_groundrules.jsp