A tip is just an investigative lead. "Hooker Chemical is dumping massive amounts of toxic waste at Love Canal" would be a tip. It would then be up to an investigative reporter to investigate, and report what the investigation uncovers.

You may be thinking of "unnamed sources" or "anonymous sources". Those people may be quoted on a factual basis, but the reporter has an obligation to obtain the same kind of confirmation of their information as any other investigative lead.