Mobile genetic elements (MGEs) are segments of DNA which encode enzymes (such as transponases) and other proteins, that allow movement of DNA. This movement can occur either between cells - intercellular motility (via transformation, transduction or conjugation in prokaryotes), or within one cell - intracellular motility. Mobile genetic elements, which mediate gene transfer by conjugation are for example conjugative plasmids, intergated conjugative elements or conjugative transposons. Transduction on the other hand is mediated by bacterial viruses - bacteriophages. Finally, intracellular movement generally involves transposons.
reference: Frost et al., Mobile genetic elements: The agents of open source evolution, Nature reviews, 3 722 - 732, 2005