Cells are broadly divided into two types: prokaryotic and eukaryotic. Prokaryotic cells tend to be smaller and simpler than eukaryotic cells. Prokaryotes are all unicellular organisms, so they rely on their cellular structures for all functions of life. Eukaryotes may be unicellular or multicellular. Multicellular eukaryotes may have many different types of cells within a single organism, each specialized to carry out a particular function, such as electrochemical signaling (e.g., neurons) or movement (e.g., muscles).
All cells have a few things in common, however. All cells are bound by a cell membrane, a semipermeable membrane (consisting of specialized proteins embedded in a phospholipid bilayer) that surrounds a cell and controls movement of materials into and out of it. The cell membrane separates the interior of the cell from its environment. Some cells may additionally be surrounded by a cell wall, which adds rigidity and affords extra protection. For example, plants and most bacteria have cells walls, which many animals, including humans, do not. Within the cell is cytoplasm, which is a combination of cytosol—a jellylike fluid primarily made up of water and dissolved substances that fill the spaces around the internal cell structures—and organelles (excluding the nucleus).
All cells also contain genetic material, which contains the instruction for maintaining the cell and carrying out its life processes. Eukaryotes, being larger and generally more complex than prokaryotes, tend to have more genetic material, although the relationship between complexity and the amount of genetic material is not linear.