Agarose (sometimes also known as "agarbiose") is a linear galactose polymer which is created by purifying agar. When it is heated and cooled, it forms a gel that is used as a support for many types of electrophoresis and immunodiffusion. Agarose gel electrophoresis is used to separate large DNA and RNA molecules (generally around 20,000 nucleotides in size).
A typical gel is about 1% agarose. Agarose is more porous than acrylamide and is sold in different grades; the lower its sulfate content, the more highly purified it is.
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/