A plasmid is a small, circular piece of DNA that is separate from the chromosomal DNA found in bacteria and some yeasts. They can replicate independently and often carry genes that confer special advantages, like antibiotic resistance.
Related terms
Transformation: This is one way bacteria can acquire new genes, by taking up naked DNA (like our plasmids) from their environment.
Conjugation: Another way bacteria share genes - this time it's more direct, like handing off the "USB stick" through a physical connection between two bacterial cells.