As the question title says it all, I read Google support, I read few things here and there, the only thing they say is it's for developers and it is updated at a rapid speed, but what way exactly it is different from Google Chrome?
Chrome has 4 releases - a regular release, a beta release, a dev release and canary. Canary is simply a much newer release thats not as well tested, but has the latest shiny stuff. After a while the version that was released in the canary channel gets any bugs that are found fixed, then filters downward to dev, and then to the beta and regular releases. Other than the lack of testing, and possibly not having all the bugs fixed, canary is merely chrome FROM THE FUTURE (except for those features that might get scrapped due to lack of quality)
In short, you get cool stuff, but it might crash horribly. On the other hand, you don't have to use it as a primary (in fact, you cannot set it as default) browser. Its mainly useful if you like living dangerously and want to test bleeding edge features
Canary is a version of the Chrome browser that is released daily, and automatically, for the Mac and Windows platforms. To get an idea of what Canary is, one can take a look at Chrome's assembly line.
Chromium is the source of the four versions of Google Chrome: Canary, dev, beta, and stable. Chromium is being continuously built and checked by an automated process, the BuildBot, which is overseen by human sheriffs, usually developers. (Chromium lacks Chrome's proprietary features.)
According to Chrome Canary for developers, "A new Chrome Canary build is available daily (we cut at 2am PST and take the best of last 40 revisions, to be specific)" and according to Google Chrome: How does the Chrome team ensure the stability of the canary channel?, "Canary Chrome contains the very latest state of our source tree, and is automatically built and shipped to users every day, usually without any human oversight at all."
However, features that appear in one day's Canary may or may not present in the next day's version and may or may not carry forward to the dev version of Google Chrome. Dev, after testing, is promoted to beta which is mostly what the stable version is after some polish. According to the cadence, while Chromium is built continuously, and Canary is built daily from Chromium builds, the dev, beta, and stable have a life of about six weeks (with bug and security fixes).
The important thing to note is that Canary is installed totally separately with its own profile and can coexist with a dev or beta or stable installation. One Chrome developer prefers having Canary and Stable:
(Canary was chosen because caged canaries were carried by miners to test the quality of air underground.)