A plasmid is a circular, double-stranded, extra-chromosomal DNA capable of autonomous replication. Plasmids occur naturally in many bacteria, archaea, and even in eukaryotes like yeast. Their size ranges from a few thousand base pairs to more than 100 kilobase pairs (kbp).

Plasmids can be classified into different types based on their functions. R-plasmids provide drug resistance by encoding enzymes that deactivate antibiotics like ampicillin, tetracycline, and chloramphenicol. Col plasmids produce toxins called 'colicins' that kill other bacteria. F-plasmids contain transfer genes that enable plasmid transfer to other host cells through a pilus. Plasmids that can integrate into chromosomes are called episomes.

Based on copy number, plasmids are classified as high or multi-copy plasmids (10-30 copies per cell with relaxed replication) and low or single-copy plasmids (one or few copies per cell with stringent replication).