Plasmids are small circular sections of DNA. These contain approximately 1000 to 1 000 000 nucleotide bases (1-100kbp).
A single plasmid usually carries between 5 and 100 genes.
Plasmids tend to code for strongly advantageous traits. For example, traits that enable the bacterium to survive in the presence of antibiotics, utilise new carbon sources or cause disease.
Plasmids can replicate and be transmitted independently.
This means that genes can be passed horizontally (between cells in a generation) as well as vertically (from parent to daughter cell).
Vertical transfer through binary fission is the primary method of plasmid inheritance.
Horizontal gene transfer allows advantageous traits to pass rapidly through populations. It also increases the genetic variation in a population, increasing adaptability.