A cell is the most fundamental unit of biological life. All known life, except for viruses, is made up of cells. Cells are also the smallest metabolically functional unit of life, meaning the smallest unit that can take in nutrients from the bloodstream, convert them into energy, perform useful functions, and excrete waste. There are two primary types of cells in the kingdom of life - prokaryotic cells, smaller bacterial cells without a nucleus, and eukaryotic cells, larger plant and animal cells with a true nucleus.
Cells are quite small. Prokaryotic cells are typically 1-10 µm (micrometers, or millions of a meter) across, while eukaryotic cells are 10-100 µm. Eggs are large single cells, and the largest known cell today is the egg of the ostrich, although prehistoric birds and some dinosaurs had eggs almost a foot in length. Every cell is produced from another cell, and each contains special genetic programming to manufacture proteins to replace things when they break down, divide, and perform the functions of life.
An aggregation of cells is known as a multicellular organism, humans being one example. These cells are so tiny and numerous, and work together so smoothly and uniformly that it took until 1839 for us to figure out that all life is made of cells. This "cell theory" is attributed to Matthias Jakob Schleiden and Theodor Schwann, German botanists who observed cells under a microscope. Soon after, Robert Hooke, the English scientist, named these little structures cells, after the Latin cellula, meaning a small room.
Another difference between prokaryotic and eukaryotic cells is the presence of intracellular machinery, or organelles. Prokaryotic organelles are quite minimal, with a plasma membrane (phospholipid bilayer) that does most of the work done by specialized organelles in eukaryotes, such as serving as the power plant of the cell and packaging macromolecules synthesized by the ribosomes. Aside from the ribosomes, cytoplasm (cell fluid), and the plasma membrane, prokaryotic cells may have another additional organelle called the mesosomes, but recent research suggests that these may merely be artifacts formed during the process of chemical fixation for electron microscopy and thus not even natural.
For some organelles in the more complex eukaryotic cells, see the article "What are some Organelles in the Cell?"