A concatemer is a polymeric DNA sequence which contains many repeats of the same DNA sequence. They are seperated by cos sites (protein binding nucleotide sequence) which only occur once in each copy of the genome. For example if the genes were arranged CAT, the concatemer would be CATCATCATCAT...etc.
Concatemers can be beneficial within DNA cloning when using gamma-phage. The concatemers contain no gamma DNA but are able to be packagedin vitro as long as a cos site is present and the correct cos spacing is available. The phage cuts the concatemer and packages it.