A sort of internal virus - perhaps a partially disabled virus. It cannot package itself into a container and escape so it is forced to hitch a ride within the host genome and occasionally get copied. For example LINE-1 (Large Interspersed Nuclear Element)is present in over 100,000 copies in mammalian genomes (http://biochem.uthscsa.edu : S.C.Hardies). The mechanism by which the transposable element moves around is provided by a protein that it codes for. In this sense it is even more minimalist than a virus; it encodes the one protein necessary for its reproduction. If two transposons flank a host gene - and if they run in opposite directions - they can cut out the gene and move it. Obviously, this jumping around can be dangerous for the host, especiallly if essential genes are interrupted.

The moving of genes by transposons is a vital part of the development of antibiotic resistance. Transposons allow genes to be moved between plasmids and the host genome, meaning that a gene carried by a bacterium that provides it with resistance can be moved to a plasmid and then transferred to other bacteria which may not have had it originally and thus gain the resistance.