Astrocytes are star-shaped glial cells located in the brain and spinal cord. They are not neurons, but instead work integratively with the central nervous system to regulate extracellular ionic concentration necessary for the transmission of neuro-electrical signals, repairing damage to nerve cells, promoting myelination of white matter, and regulating the release of neurotransmitters such as glutamatate.

Astrocytes contain Glial Fibrillary Acidic Protein (GFAP), a protein coded for by the GFAP gene that works in intercellular signaling in the central nervous system, as well as providing a functional blood-brain barrier (separating circulating blood from extracellular brain fluid). 

Sources:

http://medical-dictionary.thefreedictionary.com/astrocytes

http://www.sciencedaily.com/articles/a/astrocyte.htm

http://en.wikipedia.org/wiki/Astrocyte