A cell line is a population of cells that are derived from a single cell and are grown and maintained in a laboratory setting. Cell lines are used in research to study various biological processes, test new drugs, and investigate disease mechanisms. They offer a consistent and reproducible model system for studying cellular behavior and responses to stimuli. Cell lines can be derived from a variety of sources, including human, animal, and plant tissues, and can be manipulated and modified to study specific characteristics or genes of interest.