What is the difference between a free safety and a strong safety?

the strong safety lines up on the strong side - whichever side the tight end lines up on

The strong safety will usually defend the tight end in man-to-man defenses while the free safety will read the offense and determine where the ball is going to be thrown and try to be there to help in defending. Also, generally speaking, the strong safety will help towards the line of scrimmage on running plays.

Typically the strong safety is used more in run support and a free safety is used more to defend the pass. Strong safeties are usually slightly bigger, better tacklers, and play closer to the line of scrimmage. Some strong safeties are former linebackers like Michael Boulware of the Seattle Seahawks.