Most simply, they are two different drugs. Concerta is a sustained-release formulation of methylphenidate, and Adderall XR is an extended-release formulation of a mixture of amphetamine salts. It's like the difference between Ritalin and Adderall, which are the immediate-release forms.
Both drugs are stimulants, usually prescribed for ADD/ADHD. Methylphenidate is said to not be as strong a stimulant as amphetamine, but is still controlled as tightly.