QUESTION:
I've finished my art. Should I use a varnish or a fixative on it? Is there a difference between the two?

^{Featured art by Sophie on Strathmore 400 Series Toned Gray Mixed Media Paper}

ANSWER:
Varnishes and fixatives are two different things. 

A spray fixative is used to "fix" the drawing material (usually graphite or pastel) so that it does not smear as easily.

A varnish is used to change the sheen of the artwork and protect it from absorbing pollutants from the environment.It often is available in a variety of finishes including gloss, satin and matte. 

There is also a difference between varnishing a painting on canvas or board, vs varnishing a drawing on paper. If you are varnishing a drawing on paper, you will probably need to use a spray varnish.

When varnishing a drawing on paper, the varnish will absorb into the fibers of the paper and into the various drawing materials. If the drawing materials are dry (i.e. graphic, pastel), the drawing will smudge when the varnish is applied with a brush, which is why a spray should be used. 

You may need several coats. The spray varnish on paper is not removable, as it sinks into the paper fibers and possibly drawing materials. It is advisable to do a test with a "scrap" drawing using a spray varnish. This test scrap drawing should be on the same paper and with the same drawing materials as the artwork you want to varnish. This will give you a good indication of finished results and help you determine whether or not the drawing should be sprayed.