Cell lines are cultures of cells that can be grown and maintained in a laboratory setting. They are derived from a single cell and can replicate indefinitely under the right conditions. Researchers use cell lines to study cellular processes, test drugs, and understand diseases without needing to use whole organisms. These cell lines can be categorized into two main types: primary cell lines, which are directly taken from living tissues and have a limited lifespan, and immortalized cell lines, which have been modified to divide indefinitely. Immortalized cell lines, such as HeLa cells, are widely used in scientific research due to their robustness and ease of use.