What are plasmids? Describe their role in bacteria

A plasmid is usually a circular (sometimes linear) piece of double-stranded DNA found in bacteria that is different from bacterium’s chromosome. It carries non-essential genes that can augment a bacterium’s ability to survive in certain circumstances.
For example, plasmids carry genes that enable a bacterium to metabolise a certain type of nutrient otherwise it cannot enable to conjugate. Plasmids are used for a vast variety of experiments from expressing human genes in bacterial cells to DNA sequencing.