Target Site Duplications (TSDs) are direct repeats that occur at insertion sites of transposable elements + are thought to occur due to the filling in of the sticky ends caused from the staggered cut by transposases. They flank transposable elements and can be used to find their loci in the genome.
TSDs are usually shorter than 10bp + their pairs have high similarity. As ever in Biology, there are exceptions: TSDs of length 126bp have been reported in rotifers! 
1. Linheiro, R.S. and Bergman, C.M., 2012. Whole genome resequencing reveals natural target site preferences of transposable elements in Drosophila melanogaster. PloS one, 7(2), p.e30008. [describes the mechanism in detail]
2. Gladyshev, E.A. and Arkhipova, I.R., 2009. Rotifer rDNA-specific R9 retrotransposable elements generate an exceptionally long target site duplication upon insertion. Gene, 448(2), pp.145-150.