A real estate agent (or "salesperson" in RECO's terminology) is anyone registered with the Real Estate Council of Ontario to practise real estate in Ontario. A realtor is a real estate agent who is also a member of the Canadian Real Estate Association (CREA) and bound by their Code of Ethics.
"Realtor" is a trademarked term owned by CREA. Only CREA members can legally call themselves realtors in Canada. Non-member agents must use "real estate agent" or "salesperson" in their marketing and signatures.
In practice, the vast majority of Ontario real estate agents are CREA members and use the realtor title. CREA membership is bundled with most local real estate board memberships, and local board membership is required for MLS access. Since MLS access is essential to most real estate practice, most agents become realtors by default.
The CREA Code of Ethics is more detailed than RECO's Code of Ethics under TRESA. Realtors must follow both. In a complaint situation, both RECO and CREA can investigate, with RECO holding the regulatory power (suspension, revocation) and CREA holding the membership power (revoking realtor designation).
For consumers, the practical difference is small. Both realtors and non-realtor agents owe the same legal duties under TRESA. The realtor designation signals an additional layer of professional accountability and access to MLS, which 99% of Ontario consumers expect.