The smallest unit of life that can exist independently is called a cell. Cells are the basic structural and functional units of all living organisms, ranging from single-celled bacteria to complex multicellular organisms like plants and animals. They perform essential functions such as metabolism, growth, and reproduction, and they can exhibit specialization in multicellular organisms. Cells can be classified into two main types: prokaryotic, which lack a nucleus, and eukaryotic, which have a defined nucleus. Understanding cells is fundamental to the study of biology and life sciences.