Cells are the basic structural and functional units that make up all living organisms. There are two main types of cells: prokaryotic cells, which are single-celled organisms like bacteria, and eukaryotic cells, which form multicellular organisms like plants and animals. All cells contain a nucleus and organelles that allow different functions. Cells reproduce through cell division, which can be mitosis, which produces identical daughter cells for growth and tissue repair, or meiosis, which produces gametes like sperm and egg cells with half the number of chromosomes to ensure offspring have a full set. The cell cycle involves growth, DNA replication, and division into two daughter cells through carefully regulated phases.