Free Worms come in two varieties, in terms of the plastic used to make them. There's the Standard Formula (SF) and the Tournament Series (TS).
Standard Formula worms are fairly soft and float. Tournament Series worms are super soft, but sink.
Fish the Standard Formula in situations where you want to go weedless, or when you want the bait to stand up, say on a shaky head jig. Choose a Tournament Series version when you want to throw something like a weightless Texas rig.
All free worms are scent impregnated and produce a motion that is irresistible to fish.