Domain owner is a general term for all the millions of people who own domain names, for either personal use or business. Some use domain names to make revenue as webmasters, some use them to make revenue from PPC feeds, some use them for a family website, some do not use them at all. All of these people fall into the category of "domain owners".
Of all the domain owners, only a small percentage can be called "a domainer". Domainers are the internet enterpreneurs focusing on domain name speculation and monetization. They are the people who make a living(or partial living) from monetizing domain names, or selling them to resellers / end users.
A domainer is also a domain owner, whereas a domain owner is not necessarily a domainer.
I personally, when asked, I say that I am a "Domain name Investor"