Cell

Cells are a fundamental component of all biological existence, often referred to as the 'building blocks' of life. Some organisms, such as bacteria, are unicellular; cells exist as independent units of life. Other higher organisms are multicellular, comprised of many cells. Within these multicellular organisms, groups of specialised cells collaborate to perform specific functions (these collaborations are referred to as tissues).

All cells adhere to specific fundamental principals, irrespective of their eukaryotic or prokaryotic classification. All cells must: have a plasma membrane, to separate the intracellular space from the extracellular space; contain heritable genetic material, to ensure the propagation of the species; contain intracellular compartments, to regionalise specific functions; regulate their osmotic gradient via the transportation of chemical species (ions, compounds, sugars) across their membrane.

Cells are comprised of inner compartments, known as organelles that control the cells processes, these include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, lysosomes and vacuoles, which are all adapted to their own specific function within the cell, organelles tend to exist solitary within the cell but some exist in large quantities for example mitochondria, which is an adapation of the cell for its function, many mitochondria for a large amount of energy.  Cells within different tissue create different proteins, but all cells contain the same genetic code throughout the entire body.

REFERENCE

http://www.ehow.com/about_5079491_human-cells.html