A bacterium is an organsism. That means it’s a life form based on cells (in this case, each bacterium is one cell) which engages in the life processes of ingestion, digestion, excretion, respiration, reproduction, etc. They can grow and reproduce independent of other living matter.
Viruses are not complete cells. They cannot build themselves, and can only reproduce by invading a host organism and using some of its capabilities.
Here are some links that discuss it:
http://vm.cfsan.fda.gov/~dms/qa-fdb38.html
http://www.microbe.org/microbes/virus_or_bacterium.asp