Surfaces floats 1. Standard float that requires a stop knot and bead to lock it at the required depth. 2. High visibility floats, this also requires a stop knot and bead to set the requires depth. It has a vain in the top similar to the flights on darts. Both of these types of float can be used for float legered or float fished baits.
Sunk floats 3. and 4. These are used when you want to keep your line clear of an under water obstruction, such as weed, boulders or a drop off. (see below)
Self locking floats (polaris) Your line is fed through a hole in the cylinder at the base of the floats. The float slides up your line as the weight sinks to the bottom, when you tighten your line it locks the float in place. These are used for float leger.

This demonstrates how a standard float or sunk float can be used to avoid an under water obstruction, such as a drop off. The standard float has its depth set over the depth of the water, the sunk float has its depth set at less than the depth of the water but high enough to clear the obstruction.