Larva
Dr. Gero's super computer created this small android from the cell's of the earth's greatest fighters. Cell spent years of isolation in this form while the computer programmed data into his brain.

Egg
His "shell" form (seen below) spawns from this egg. Bulma, Gohan and Krillin find it. They soon realize something very powerful is about to attack earth...

Shell
Cell breaks out of his egg, and turns into this.We don't really know how much power he has in this stage, he just has to de-generate to it in the alternate timeline. We never see this stage in the present.

Imperfect Form 1
This is the first form we see Cell in. His horns are extending outwards, instead of upwards like in his later forms.He absorbs people by piercing them with his sharp tail which allows him to suck the entire body: skin, fluids brain and all, into himself. He does this to tons of innocent humans.

Imperfect Form 2
After Cell absorbs #17, he reaches this form. Cell's power increases dramatically and he gets a human-like mouth, too. When Vegeta battles Cell, his pride dominates him and he allows Cell to absorb #18. Which takes us to his "Perfect" form.

Perfect Cell
As I said above, after he absorbs #18 he turns into this form. This form is so powerful that most of the Saiyans can't even injure him! He gains a lot of familar attacks (Special Beam Cannon, Kamehameha, etc). Gohan in his Super Saiyan 2 state is the only person who really has a chance against this machine.

Perfect Cell Full Power
Cell gets so engraged that he turns into this form. He has a golden aura around him, and has USSJ2-like abillities (muscle mass growth). This was the same mistake that Trunks made, now Cell has a dramatic drop in speed which makes him easier to hit for Gohan.