Safeties
The free safety (FS) is responsible for reading the offensive plays and covering deep passes. Depending on the defensive call, the safety may also provide run support. The safety is positioned 10 to 15 yards behind the line of scrimmage, depending on the safety's speed. There are usually two safeties per play, which are placed on opposite ends of a formation. However, if a defensive play may only require one safety, the player is usually positioned in the center of the formation. The safety provides the final line of defense against running backs and receivers who elude the linebackers and cornerbacks. Usually, a safety is agile, dexterious, vigilant, and capable of predicting the offense's next play just by looking at their formation.
The strong safety (SS) is usually larger than the free safety, and is positioned relatively closer to the line of scrimmage. While strong safeties play a pivotal role in halting the offense's running game, they are also responsible for keeping various receivers, especially tight ends, in check.
http://en.wikipedia.org/wiki/Defensive_back
Too lazy to type it for you