Surprise! They are the same thing - Canvas (made by a company called Instructure) is the commercial name of the learning management system and Quercus is how it has been branded at the University of Toronto. You'll hear the system referred to interchangeably, but we do try to use Quercus (or "the Q").
Other institutions use Canvas by Instructure, but no other school will call it Quercus. If you're doing a search of community forums, for example, you'll want to use Canvas as your search term, not Quercus.