The free safety and strong safety are both members of the secondary.
The secondary is a group of players in the defensive unit that typically stand in the defensive backfield or the area behind the line of scrimmage and the defensive line. The safeties serve as a last line of defense between the opposing team's offense and the end zone. Safeties can also be used to blitz or defend the run.
The differences between a strong safety and free safety are based on the following:
The free safety positions himself on the weak side of the field.
The strong side is the side of the line that the tight end lines up on. The other side of the field is called the weak side. The free safety is generally smaller than the strong safety and slightly faster. Free safeties also generally inherit more pass coverage than strong safeties.
The strong safety is the player located on the strong side of the football field. The strong safety is generally bigger and stronger than the free safety, and is used slightly more for run defense.
Regardless, both the free safety and strong safety are important players on a football team.