The basic unit of life that all organisms are made up of is a cell. Cells are the smallest structural and functional units of living organisms, capable of performing vital processes necessary for life. They can be classified into two main types: prokaryotic cells, which lack a nucleus (e.g., bacteria), and eukaryotic cells, which have a nucleus and organelles (e.g., plant and animal cells). Cellular organization is fundamental to biology, as it underpins the complexity of living systems.