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.