Plasmids are small, circular DNA molecules that are separate from the chromosomal DNA in bacterial cells. They can replicate independently and are often used as vectors in molecular biology research to introduce new genes into bacterial cells. Plasmids can carry genes that provide antibiotic resistance, allow bacteria to metabolize certain nutrients, or produce useful proteins. They play a key role in biotechnology and genetic engineering, as they can be used to manipulate genes and study gene function. Plasmids are also commonly used in the production of recombinant proteins and genetically modified organisms.