In this form Cell had no power because he was still in the test tube created by Dr. Gero.
2. - Huge Bug
In this form cell can burrow and shed its skin to its next form. In this form he is not in the test tube.
3. - Imperfect Cell1
In this form, Cell is very powerful. Stronger then a supersaiyan. He battles Piccolo{Super Namek} in this form.
4. - Imperfect Cell2
In this form, Cell has absorbed Android17 and is much more powerful then before. His tail is longer also.
5. - Perfect Cell
In this form, Cell has absorbed both of the androids and he is complete. He is stronger then anyone except Ssj2Gohan. He has no tail in this form. Only his stinger.
6. - Ultra Perfect Cell
This form is same as before but he is even stronger. He reached this form when he self destructed on King kias planet. One of his cells remained and that is all he needed for his regeneration. After he totally healed he was super strong because he had saiyan cells in him and once a saiyan heals from battle they are even stronger.
Ok that is all the forms. There is another form were he is in an egg but that was only to get in the time machine.