There are two different kinds of organisms. The first kind only have one cell and are known as unicellular organisms. The second kind of organsim has more then one cell and are known as multicellular organisms.
Unicellular organisms have only one cell. They are also very simple organisms. Mitosis is how they reproduce. This is called ASEXUAL REPRODUCTION.
Multicellular organisms have many cells or only a few but always over one. These organisms are complex. They have SEXUAL REPRODUCTION.