Cells

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The word cell comes from the Latin cellula, meaning, a small room. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.

The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

Living cells are dived into two types - procaryote and eucaryote (sometimes spelled prokaryote and eukaryote). This division is based on internal complexity.

Eucaryote:

Animals, plants, fungi, and protists are eucaryotes, organisms whose cells are organized into complex structures enclosed within membranes. The presence of a nucleus gives these organisms their name, which comes from the Greek (eu), meaning "good/true," and karyon, "nut." Many eukaryotic cells contain other membrane-bound organelles such as mitochondria, chloroplasts and Golgi bodies. These cells tend to be larger than cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size. Use the interactive animation of plant and animal cells to learn about their respective organelles.
http://www.cellsalive.com/cells/cell_model.htm


Procaryote:

The prokaryotes are a group of organisms that lack a cell nucleus, or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles.The word prokaryote comes from the Greek (pro-) "before" + karyon "nut or kernel", referring to the cell nucleus.

The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.