A plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA. In many cases, it is circular and double-stranded. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms. Plasmids are considered transferable genetic elements, or "replicons", capable of autonomous replication within a suitable host.

A chromosome is an organized structure of DNA and protein that is found in cells. A chromosome is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

The major general differences include:

• Plasmids have much less base pairs than chromosomes
• Plasmids are rarely organized by chaperone proteins
• Plasmids are easily transferred
• Plasmids usually contain non-essential genes
• Plasmids function can be lost or gained without harming the organism
• Plasmids are usually found in "lower" organisms