FRAA -- The biggest difference between Fielding Runs Above Average and similar defensive metrics comes in the data and philosophy used. Whereas other metrics use zone-based fielding data, Fielding Runs Above Average ignores that data due to the numerous biases present. Fielding Runs Above Average instead focuses on play-by-play data, taking a step back and focusing on the number of plays made compared to the average number of plays made by a player at said position. The pitcher's groundball tendencies, batter handedness, park, and base-out state all go into figuring out how many plays an average player at a position would make.
Here is an example of the Fielding Runs Above Average spectrum based upon the 2011 season-for the sake of consistency, the players featured below all play the same position (center field):
Excellent - Jacoby Ellsbury 11.6
Great - Nyjer Morgan 5.5
Average - Marlon Byrd 0.6
Poor - Roger Bernadina -5.2
Horrendous - Melky Cabrera -13.2
WARP components can be found in this article, which also describes 2015 changes to FRAA: http://www.baseballprospectus.com/article.php?articleid=27944