Assume an experimental target that is to make vast amounts of a particular DNA fragment in pure form from a combination of DNA fragments.  Whereas the DNA fragments can be introduced into bacterial cells the most or all will lack the ability for self-replication and will fast be lost. Furthermore, two kinds of DNA molecule are known that can replicate autonomously in bacterial cells; plasmids and bacteriophages. Plasmids are little circular double-stranded DNA molecules which exist free inside bacterial cells, frequently carry certain genes which confer drug resistance and are self-replicating of the genes.  If a recombinant DNA molecule is establish by joining a foreign DNA fragment to plasmid or bacteriophage DNA then the foreign DNA is replicated when the plasmid or phage DNA is replicated.  In that role the plasmid or phage DNA is known as a vector.  Todays, a population of recombinant DNA molecules can be build, every recombinant molecule having one of the foreign DNA fragments in the original combination.