A cell line is a population of cells that can be cultured and maintained in a laboratory setting. These cells are derived from a single cell and can replicate indefinitely under the right conditions. Cell lines are essential for various research applications, including drug development, cancer research, and genetic studies. Cell lines can be classified as either primary or immortalized. Primary cell lines are directly taken from living tissues and have a limited lifespan, while immortalized cell lines, such as those derived from the HeLa cells, can divide indefinitely. Researchers use these cell lines to study cellular processes and test new treatments.