What is the difference between a canned response and a signature?

A signature is personal: each User creates their own under My profile > Signatures and adds it as the sign-off of a reply. A canned response is shared by the workspace: owners, admins and group admins create them under Configurations > Canned responses and any User can insert one as the body of a reply. In short, signatures sign a message and canned responses answer it. Both have their own button in the actions row under the reply editor.