in all actuality....the "name" of the position explains it. for example:
FS = Free Safety. this cat is just that...FREE!! he covers his half of the field but also has the "liberty" to roam.
SS = Strong Safety. this cat is the one that is (imo) feared the most because of his tackling ability in the run support. he is the one that lines up closer to the line.
Ed Reed (FS) is not really feared for his "hitting" or "tackling" ability....but....cats don't want to throw the ball anywhere near him cause he is a pure ballhawk and it maybe a pic for 6!
Troy Polomalu (SS) is mostly feared for his "tackling" and "hitting" ability...so cats don't want to go anywhere near him because it may be a instant fumble when he hits you.