A short sequence of DNA (known as a transposon) which can change location on the bacterial genome (the sum total of all of the bacterium's DNA) and contains genes which code for proteins that enable it to change location.
They are useful because they can also contain genes for other things, like antibiotic resistance, and because they can be introduced into a bacterial genome by a researcher.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.