Plasmid
A plasmid is a small ring of DNA inside bacteria that carries extra genes.
Think of it this way
A kitchen cupboard holds a large recipe book — that is the main DNA. Next to it sits a small index card with a few bonus recipes on it. That index card is the plasmid. Bacteria can copy and share these cards with each other, passing along the bonus recipes.
Explaining plasmid by grade level
Bacteria are tiny living things. Inside them are even tinier rings. These rings hold bits of code. The code can give bacteria new skills. One skill might make them glow in the dark. Scientists can move these rings from one germ to another.