When Cell first came to be he was just a tiny and harmless organism sleeping in a giant tube and being fed data by Dr. Gero's super computer, gradually growing and gaining strength.
 Insect Shell
Eventually Cell was finished being worked on by the super computer and emerged from the secret laboratory as this state that looks like a giant insect. Cell doesn't remain in this state for long though, and soon sheds this outer skin to change into his Imperfect form.
Tall, menacing, and with distinct insect-like features, Cell becomes a menace to the Earth due to his incredible power and ability to absorb lifeforms through his tail, making himself stronger.
 Imperfect Form 2; Semi-Perfect
Upon absorbing Android 17, Cell transforms into his semi-perfect state. He is bulkier, and looks somewhat more human than he did beforehand. He becomes a lot more powerful as well, even though he was no match for Vegeta after his training in the Hyperbolic Time Chamber.
After finally managing to absorb Android 18, Cell achieves his perfect form, becoming shorter with less muscle mass and looking more humanoid than ever. His tail is now noticeably shorter than in his previous forms, but the stinger tip is still present on his back. In Dragonball GT however, it is revealed that Cell can in fact still extend his tail when he attempted to absorb Goku.
 Perfect; Full Power
Fighting with Super Saiyan 2 Gohan eventually enrages Cell to the point of pushing his power to the limit. Becoming much taller and unbelievably more bulky, this state causes Cell to lose loads of his speed due to the considerable size of his muscles, rendering this state quite useless against his opponent.
 "Super" Perfect
After regenerating from his own self-destruction, Cell evolves into a state looking the same as his perfect form, but with an large increase in strength. This state mimics the Super Saiyan 2 transformation in that it now also has lightning currents blazing through its aura.