"What exactly is plasma?"

You can see the light that is given off from plasma.  That is how you know it is there.  The Sun is a big ball of plasma.

You can measure plasma in many ways.  You can measure its temperature, and the strength of the magnetic field it generates.  On Earth, scientists make man made plasmas, using a known amount of specific elements, like Neon.  Neon lights are a form of plasma.  You cannot see the gas, but you can see the light generated from the excited gas.

Ancient philosophers classified everything as Earth, Water, Air, or Fire.  Some think this stands for solid, liquid, gas, and plasma.  The first person to experiment with plasmas was W. Crooke in the late 1800's, by arcing electricity through low density gases.
http://www.madsci.org/posts/archives/2000-11/973706828.Sh.r.html

Plasmas give off light and generate a magnetic field.  Other than that, they don't do much, just like solids, liquids, and gases don't do much on their own.

The plasma tv screens have tiny chambers of neon and xenon for its pixels.  These chambers are bombarded with UV light of different intensities as the pixel color changes.  The gas turns to plasma and emits light based on the energy of the UV light that excited it.
http://wiki.answers.com/Q/How_does_a_plasma_TV_work

I hope this helps.

know if plasma TV use plasma... but fluorscent lights do.