release the wire

Every wire basically consists of one or more conductors and insulation material around it to shield the conductor from short circuiting with another conductor.
Conductor thicknesses are standard, they can be in accordance with the american wire gauge, circular area units, diameter, etc.
More information about wire thickness and other wire/pins/sockets related information can be found on my colleague Jan's website pins and sockets

Basically the higher the AWG number, the thinner so a AWG 24 being very thin and a AWG 16 being pretty thick and suitable for more ampere's.

For this subject I will use stripmaster wirestrippers.
The head on the wire stripper holds the cutting blade sets, the numbers next to the holes correspond to the size of the conductor of the wire that needs stripped.
AWG sizes
picture by: http://aeroelectric.com/

The wire that needs to be stripped is lined up with the correct size hole.
wire lined up

Then the handle is squeezed, causing the blade to cut and pull off the insulation material.
isolating material removed

At this point, the handle should be released a little bit to release the block clamp that is holding the wire.
release the wire

If the blades are released without the wire being removed from the stripper the stripper would snap closed and damage the wire.
remove the wire from the stripper

A properly stripped wire should look something like this.
properly stripped wire
picture by: http://aeroelectric.com/

A wire stripped with a lesser tool usually looks more like this.
cheaper tool
picture by: http://aeroelectric.com/

There are different types of wirestripper blade sets.
wirestrippers

Here's a blade set that is used for fire detector wiring, this wiring is heat resistant and has a protective sleeve under the insulation material.
fire detector wire stripper

A more standard wirestripper has round holes and the insulation cutting blades vary in pitch, here's one with a very dull pitch.
dull pitch

And here's one with a very sharp pitch.
sharp pitch