Upon the discovery of the cells in the 17th century, they devised the theory that sum up the description of cells. Initially, they had three claims:

• All living things or organisms are made of cells and their products.
• New cells are created by old cells dividing into two.
• Cells are the basic building units of life.
  source: http://en.wikipedia.org/wiki/Cell_theory and http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Cell_theory.html

This list has increased and more specified as the advancements in the technology increases. The following are some of the ideas that are added in the cell theory. Some of the thoughts are revised to make the theory specific. 

• All living things are made up of one or more cells. The original version did not mention that unicellular organisms exist.
• All living cells arise from pre-existing cells by division. 
• The cell is the fundamental unit of structure and function in all living organisms. - Revised version of the third theory
• Cells contain hereditary information (DNA) which is passed from cell to cell during cell division. - All cells contain genetic information which is being inherited by the daughter cells after cell division.
• Energy flow occurs within cells. -Cells metabolize. This means that they use energy in order to perform specific function. 
• The activity of an organism depends on the total activity of independent cells. 
• All cells are basically the same in chemical composition in organisms of similar species.
  Source: http://en.wikipedia.org/wiki/Cell_theory