The cell is the basic unit of life. Every organism is either made up of large interconnected groups of cells or is itself a cell, free-floating and independent. Naturally, given the diversity of life that exists on earth, there is a nearly endless variety of cell types. The human body alone contains more than two hundred different kinds of cells that vary in size, shape, and function.
Biologists generally classify all cells into two different categories, depending on the presence or absence of a nucleus. Cells that lack a nucleus are called prokaryotes; cells that have a nucleus are called eukaryotes. The majority of prokaryotes are bacteria. The DNA of these cells is contained in a single chromosome that floats freely in the cell's cytoplasm. Nearly all other cells, including all plant and animals cells, are eukaryotic, and thus keep most, if not all, of their genetic material in a nucleus.
Cells of different types vary widely in size. Generally, prokaryotes, which range from 1 to 10 microns (thousandths of a millimeter) in diameter, are smaller than eukaryotes. One E. coli bacterium, for example, measures only about two microns in diameter, even smaller than a single mitochondrion, a eukaryotic organelle. Most eukaryotic cells average about 20 microns, while some cellular giants, including human egg cells, measure 100 or more microns across, the diameter of a typical human hair.
At first glance, the shape of most cells is fairly ordinary. Many animal cells are spherical; plant cells are typically boxy. But if you look more carefully, you'll see that a cell's shape often relates closely to its role in the organism or its behavior in its environment. The outermost cells in your skin, for example, are flattened and overlap each other like shingles on a roof. And like a roof, your skin protects your body from the environment. Neurons are even more specialized. Like most cells, neurons have a spherical cell body with a nucleus inside. But neurons also have threadlike projections, called dendrites and axons (some axons are three feet long!), that transmit nerve impulses throughout the body.
Genetic information contained in the DNA in every one of our cells tells those cells how to behave by dictating which proteins cells should create and when cells should create them. And amazingly, thanks to the process of DNA replication, all of this detailed information can be passed not only from parent to offspring but from "parent cell" to "daughter cell" whenever the parent cell divides.