What are plasmids? How does transfer of genetic materials take place by employing plasmids?

Plasmid is a circular, extra-chromosomal DNA present in some bacteria.

Plasmids are used as a vector for carrying out transformation experiments since it contains an origin of replication(ori site, essential for initiation of replication) and other antibiotic resistance genes that aid in identification of transformants.The desired gene is inserted in the plasmid and multiple copies of Plasmids are obtained which are further introduce in the host cell(for e.g.- E.Coli).