Definition of float
A float is usually done in position. The player who floats calls a player's bet in order to bluff him later in the hand. The floater may have a marginal hand with little room for improvement. But he calls in position because he knows the player he is playing against is face up or because he thinks he can bluff him later. Don't mix up float and chase. When you chase, you call with the goal of hitting your draw. When you float, you call with the intention of bluffing later in the hand.
Is it a float or the nuts?
A good player who floats will be hard to beat. A float is played as if you had the nuts (the best possible hand). If you have 99 out of 992, turn 5 and river 6, you will probably call the flop, call the turn and raise the river. If you're in the float, you'll play exactly the same way, but bluff. By calling, you keep your range bigger in your opponent's eyes. And if a draw hits, you can still represent it if you were bluffing. You won't see much float at the low limits, but more at the mid and high limits.