Safeties

Safeties along with conrerbacks are DB's (defensive backs). The safety (S) has multiple job responsibilities on defense. They can play man to man, zone or free. But more importantly, they are the last line of defense to stop the offense because normally they are the furthest players back. To start the play most safeties start 10 to 15 yards off the ball and make adjustments depending on their assignment.

Two types of safeties:

Strong safety (SS): The strong safety plays on the strong side of the field or the side that has more offensive players. There job is to either guard a receiver, running back, or zone.

Free Safety (FS): The free safety plays on the weak side or the side with fewer offensive players. Often times they are free to move where they want depending on how the play is developed. For example if the QB starts to look at the receivers to his left then the FS will start to go that way.

http://en.wikipedia.org/wiki/Safety_(American_and_Canadian_football_position)

In the image above, the defense is in a 3-4 alignment because there are three linemen and four line backers. Cause there are only two corners there are two safeties to make 11 players. Because the TE on offense is on the right side of the formation the play is strong right. When the play is strong right, the SS is on the right while the free safety is on the left.