A cell is the basic unit of life, serving as the building block for all living organisms. Cells can be classified into two main types: prokaryotic cells, which lack a nucleus and are generally simpler, and eukaryotic cells, which have a nucleus and more complex structures. Each cell is surrounded by a cell membrane that controls the movement of substances in and out. Inside a cell, various components called organelles perform specific functions. For example, the mitochondria generate energy, while the ribosomes synthesize proteins. Cells can replicate through a process called cell division, allowing organisms to grow, repair, and reproduce.