Cell can mean different things, depending upon which specific discipline of science we are talking about. In biology or microbiology, we refer to cell as the basic structural unit of all the living organisms. Our bodies are made up of cells and so are the bodies of animals and plants. Other commonly known cells are bacteria, some of which cause disease (pathogenic microbes).

In other common usage, cell is used to mean a battery, which is a device that is used to power devices such as clocks, torches, cell phones, etc. These cells are based on chemical reactions, which may or not be reversible. For reversible reactions, electricity has to be supplied to recharge the cell. If reactions are one way reactions, cells can only be used one time and once they are discharged, they cannot be reused.

There can also be other usages of cell, such as the box in which we add values in Microsoft Excel program, etc.

Hope this helps. 

what is a cell?

A cell is the smallest structural and functional unit of an organism.  That statement is one of three parts to cell theory.  Cell theory states that all living organisms are made from cells.  All cells come from pre-existing cells.  And the cell is the basic structural unit of living things. 

Living things could be divided into 2 broad cell categories.  Unicellular organisms (bacteria, many protists, some fungi) or multicellular organisms (lions, tigers, bears -- oh my).  A multicellular organism (like a human) can be paired down into smaller and smaller chunks.  An organism is made of organ systems, which are made of organs, which are made of tissues, which are made of cells.  That's the final functional unit.  Sure, cells have things like endoplasmic reticulums and golgi bodies, but those things do not exist on their own outside of cells.  The cell is the smallest and simplest unit of living things.  

Cells themselves can be subdivided into 2 broad categories.  Prokayrotic cells or eukaryotic cells.  Eurkaryotic cells have a nucleus and cell organelles (cell organs).  Prokaryotic cells do not have organelles or a nucleus.  They are basically an outer cell membrane, a cytoplasm, and some free floating DNA.