Imperfect Cell 1st form:
This form of Cell has just come out of the larva stage and has to depend on the emergence or absorbing humans. He is more powerful than a super saiyan and is first seen in Nikki Town while fighting the new and improved super namek Piccolo. He hasn't absorbed androids 17 and 18 yet.
Imperfect Cell 2nd form:
In this form Cell has absorbed android 17 and his power has greatly increased. He has more speed and more strenght, also his appearance has gotten differnt for the worse. His tail is now longer also. he is first seen in this form while Piccolo and the androids were fighting Cell on the little island. He would have absorbed 18 but Tienshinhan came and interfered and Krillin saved her.
This is one of Cell's most powerful forms. In this form he has succesfully absorbed both androids. He no longer has a tail in this form. In this apperance is achieved he takes on a new appearance more like human in this form and his power greatly increases.
Giant Perfect Cell:
In Cell's frustration with how to defeat super saiyan 2 Gohan, he grows into a large fighter with huge muscles, putting all of his strenght down onthe board and getting rid of a lot of his speed. Gohan takes him down easily, kicking him in the chest and making Cell puke of 18 turning him back into Cell 2nd form.
Ultra Perfect Cell:
This is Cell's last and most deadly form. After destroying King Kiao's planet he regenerated from one cell, Picolos regeneration cell. He uses the instantaneous transmittion to get back to Earth as his new powered form from death. He had lost 18 but his body copied her data, allowing him to stay in Perfect form. Gohan was no match for the new Cell, but killed hum after firing the biggest and strongest kamehameha ki blast ever.