What is in a Prescription of IB that is different from OTC othere than the dosage per pill?

Could be different strength or a completely different drug. Tylenol for example, over the counter I believe it is acetaminophen but Tylenol 3s are codeine. So it might be a stronger version or a completely different drug.

Just the dosage.
If you don't have insurance that covers prescriptions, just buy OTC.

Aren't ibuprofen and its generic both OTC?

Yup. Prescription strength is just higher does. 400,600,800mg.