Hello,
In addition to CIPO, which is Canadian specific, I would recommend using the global trademark search provided by the World Intellectual Property Organization (WIPO) at
Global Brand Database
It has a much cleaner interface (imho) and better search/filtering capabilities, and it is global. However, Canadian information may be a week or two behind CIPO database.
Trademarks are dependent upon the industry/service (called the NICE classification). That is why two different companies can have a trademark using the same word. For example, Apple Computers and Apple Auto Glass. They provide different products/services but both have the word 'apple' in their trademark and there is no conflict.
Additionally, you can own and use a trademarked name in a domain name so long as you are not violating a registered trademark. For example, if you owned chevrolet.com (or .ca!) you could sell computers under that name (as long as there is no "Chevrolet" computer company with a Canadian trademark on that word), but you could not use it for your used car sales business.
The type of trademark is very important. With domain names, we are really only interested in 'word' trademarks. However, trademarks can be other things also, such as logos (e.g; Coca-Cola logo), sounds (e.g; MS Windows startup sound), etc.
Also, beware of similar sounding trademarks. A very interesting read about a Canadian legal trademark case can be found
here.
It is important to note that a trademark registration does not automatically grant you rights to the domain name. Also, trademark searches do not typically include business names unless the business name is trademarked also.
For completeness, I would recommend doing similar searches - chicken, soup, "chicken soup" and chickensoup.
Note that I am not a trademark expert; just a guy who has done some research on the topic. For authoritative trademark information I would recommend consulting a professional trademark lawyer.
Hope this helps!