A stringer is a freelance journalist.

A stringer is a reporter who works remotely from the main office or bureaus of the news outlet. For example, the NYtimes might not have a bureau in Little Rock, Arkansas, but they might have a "stringer" there who covers stories for them when something pops up.

But the reporter would be called a stringer only if he was freelance. Stringers work for any news media as and when they are required. Good ones are proactive rather than reactive.