Plasmids are defined as autonomous elements, whose genomes exist in the cell as extrachromosomal units. They are self replicating circular duplex DNA molecules, which are maintained in a characteristic number of copies in a bacterial cell, yeast cell or even in organelles found in Eukaryotic cells.
These plasmids can be single copy plasmids that are maintained as one plasmid DNA per cell, which are maintained as 10 to 20 genomes per cell. There are also plasmids which are under relaxed replication control, thus permitting their accumulation in very large numbers.
These are the plasmids which are used as cloning vectors, due to their increased yield potential.