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, making them essential tools in biomedical research. There are two main types of cell lines: 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. Examples of immortalized cell lines include HeLa cells, derived from cervical cancer cells, and 293T cells, used in various experiments.