Plasmids are found in bacteria cells and are circular pieces of DNA that are seperate from the bacterial genome. They have the ability to replicate independently of the host cell, however their replication is dependent on particular enzymes being present in the host cell. Whilts most plasmids are circular, not all of them are. 
- ↑ Hartl and Jones
Hartl and Jones, 2009. Genetics: Analysis of Genes and Genomes. 7th Ed.