A freelance reporter or photographer that either goes to where news is happening faster than the employees of the local newspaper, TV or radio station and gets good early footage or copy of the situation. They then sell it to the local news for some arranged price to make their living.
Sometimes stringers have a good rapport with the local media and are called by the news manager, editor, or producer to cover certain events when the in-house reporter staff is busy with other assignments.
If you ask a police officer or fire fighter what they think of stringers, you usually won't hear many good words. Stringers often sit in their houses with a radio scanner tuned to the local dispatch channels and respond in their cars right along with the first responders and rescue personnel. More often than not, they respond in a dangerous manner to the scene, and being freelance, don't care about being obnoxious, getting in the way or causing problems for the officials trying to quell the situation. They just want a good story or camera angle.