Difference between needing a filling and a root canal

Fillings: To treat a cavity your dentist will remove the decayed portion of the tooth and then "fill" the area on the tooth where the decayed material once lived.
Fillings are also used to repair cracked or broken teeth and teeth that have been worn down from misuse (such as from nail-biting or tooth grinding).
First, the dentist will numb the area around the tooth to be filled with a local anesthetic. Next, a drill, air abrasion instrument, or laser will be used to remove the decayed area. The choice of instrument depends on the individual dentist's comfort level, training, and investment in the particular piece of equipment as well as location and extent of the decay.

Root Canal: The space within the root of a tooth. It is part of a naturally occurring space within a tooth that consists of the pulp chamber (within the coronal part of the tooth), the main canal(s), and more intricate anatomical branches that may connect the root canals to each other or to the surface of the root.
It is also a colloquial term for a dental operation, endodontic therapy, wherein the pulp is cleaned out, the space disinfected and then filled.