A rumor is a story which may not be true. Everyone may be talking about the rap superstar who stopped for ice cream in your town, but until there's proof that it really happened, the whole thing is just a rumor.
Are rumors ever true? Of course — sometimes word gets out ahead of time, like when a student overhears teachers talking about the early dismissal before it is announced. When the school closes early, the rumor is confirmed. But many other rumors can never be confirmed, so they stay just that, rumors. True, false, semi-true: who knows? The Latin word rumorem, or noise, is the origin; noise is often all that a rumor is.