To constrain someone by force or by social, moral or legal means.
I am obliged to report to the police station every week.
To do (someone) a service or favour (hence, originally, creating an obligation).
He obliged me by not parking his car in the drive.
To be indebted to someone.
I am obliged to you for your recent help.
Under an obligation to do something for someone.
Indebted because of a favor done.