The cell theory states:
1.- A cell is the smallest living thing.
2.- All living organisms on Earth are divided in pieces called cells.
3.- New cells are created by old cells dividing into two.
There are many types of cells. The thing they all have in common is that they are compartments surrounded by some type of membrane, the inner is the cytoplasm and the main organelle is the nucleus.
There are two types of cells:
PROKARIYOTIC AND EUKARYOTIC CELLS
Follow the links about the cell's parts:
MEMBRANE: How is the transport trhough the membrane?