What is this next to the 100% on iPhone 4?

On my iphone next to the 100% there's like a little bar and halfway it's green with a grey/black ball at the top.
What does that mean?

That's your battery indicator. The grey/black "ball" that you see is a plug icon. When you disconnect the charger or remove your phone from a dock you may have it connected to, the plug icon goes away and all that's left is a green bar. The bar indicates how much battery life you have left (even though it shows you the percentage).