For a full definition of what a dimmer is see Dimmer.
In show lighting a dimmer is used to convert DMX signals into a variable voltage to control traditional lighting fixtures such as Par Cans and others that do not have any form of DMX control. They come with a varying amount of dimmers, 1 upwards. Ones that have more than 4 dimmers have chasers built into them.
Some examples of DMX controlled dimmers are shown below. As you can see they come in a variety of shapes and connection options. The third picture is a T-Bar which you would plug and hang your Par Cans from.
Sometimes people refer to the sliding fader on a control desk as a dimmer because that is originally what it did, but in fact all it did was to tell a dimming device to perform the dimming function.