First discovered in the 1950's by Barbara McClintock, transposons were originally called jumping genes. No matter what you call them, transposons are mobile pieces of DNA that can move from one location to another within a genome. For example, when the enzyme transposase is activated, a transposon located on chromosome number 1 could jump to a locus on chromosome number 3, or X or any other locus in the genome. Because the enzyme transposase is required for its mobile capacity, investigators can control the expression of enzyme and thus the ability of a transposon to jump again or not.
Many times it is especially useful to construct a transposon that contains features that can be used by investigators. Mike Snyder's group (Proteomics Chapter) used a modified transposon to create mutants since a transposon that inserts within a gene can disrupt its ability to encode a functional protein. Many times a transposon will contain a reporter gene such as lacZ which encodes the enzyme b-galactosidase or GFP (green fluorescent protein). Other features can be added such as epitope tags and the loxP/Cre recombination system to excise from the genome large portions of the transposon.