Rust can be a terrible thing for a homeowner. It will eat through your metal roof and siding, it will cause the rebar in your cement to swell and crack the cement, it will make the nails in your walls and roof pop out and cause discoloration where the rusty water leaks out.
Rust is caused whenever water and air combine with the metal to create oxidation. You might not even be aware of how many things in your home are made of metal or held in place by metal until the rust starts showing.
Metal is usually protected from rust by a protective coating, but this can wear off over time, or damage can remove it from all or part of a metal surface. Once the rust has gotten a foothold, however, it can spread quite quickly. The sooner you deal with the problem, the more likely you are to be able to save the metal in your home.