These are used primarily for controlling volume or tone, although they can also be substituted for switches to produce variable effects (i.e., slightly "on"/slightly "off").

These usually combine a standard potentiometer with a DPDT (double pole-double throw) switch.  This means that you have a pair of switches that can each select between two things.  However, you can ignore the lugs on one side of the switch and regard it as a SPDT (single pole-double throw) or ignore all but two of the lugs and regard it as a SPST (single pole-single throw) switch as when you coil-tap a humbucking pickup.  You can read more about how switches work on the switches page.

The advantage of this is that you can use the top portion as a volume or tone knob and the switch portion for another independent purpose, such as to tap a coil, reverse the phase of a pickup or coil, or for series/parallel switching.

These are stacked potentiometers, and are useful for putting a volume and tone knob together to save space either to give a more spartan appearance or to leave room for more controls.  They can be difficult to come by since most standard guitars do not use them.  However, they are available from Warmoth among other places.

Note: While they superficially resemble blender potentiometers (aka dual-ganged pots), the separate elements (i.e., top and bottom) may be operated independently of one another on concentric pots.