Pianos have a limited lifetime and so does their working parts.  Regular replacement of worn parts can extend the life for decades to come.  Every piano will require different attention.  In some cases, the parts of the action may remain in good condition while other parts are able to be restored or rebuilt depending on the component.  

This may include: the strings, pinblock, bridges, soundboard, ribs, hammers and action.  “Restoration” implies more replacement work than mere “repair” or “maintenance,” and “rebuilding” implies more intensive work than restoration.  More over, restoration is usually more labor-intensive and is usually done to original pianos of high quality, or to those with historical and sentimental value.