Unpaid internships are volunteer work programs. People work for no pay in order to gain experience in their desired field so they may enter that field, for pay, with the gained experience.
Recently there have been lawsuits filed by people who have gained experience through unpaid internships who now want to be compensated for that work.
These people apparently place no value on the time spent learning how to work in their field. This will eventually kill the internship opportunities available, since companies won't want to be exposed to possible litigation.
But back to the topic ...
So what would qualify as an unpaid job to you?
And the aforementioned people who violate the contract don't invalidate it.