So I have the droid three now. im pretty new to all this but i want to learn anyone wanna help?

I know that most of you will say its not true but i have seen it happen. A droid 3 from verizon but with an at&t carrier SIM within it.

I have rotted my device and don't really know what its used for? also i have it unlocked to change from Global/CDMA/GSM/UMTS. at&t uses GSM.

Can someone please explain to me how this all works what i can do. Link on a tutorial how, or any bit of advice??