The three basic tenets of the cell theory are:
1.) All living things are made of cells
2.) Cells are the smallest structural and functional unit of living organisms
3.) All cells come from preexisting cells.
The first is self-explanatory, as all living things - from microscopic organisms to the largest whale - are made of cells. These cells can vary widely, but all living things are composed of cells.
Secondly, living organisms cannot be broken down any farther than the cell level. Like atoms and matter (atoms are the smallest units of matter), cells and living organisms are related: living organisms, at their most basic level, are made of cells.
Finally, no cell comes about in a vacuum. All cells on the planet came from other cells. Cells divide by a number of mechanisms, such as mitosis (non-sexual cell division) and meiosis (sexual cell division), but every cell in existence came from another cell.