The cell (from Latin "cella", meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.

All cells have a membrane that envelops the cell, regulates what moves in and out, and maintains the electric potential of the cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment.

More Info: en.wikipedia.org