The Canary build is only available for Windows, it's "installed to a different path, gets updated separately, and runs side by side with an existing stable/beta/dev installation". Google says that the Canary build will usually be the same as the dev build. "Sometimes if necessary, we may push additional updates on Canary build so its version is higher than dev."
"The canary usually updates more frequently than the Dev channel (higher risk
of bustage), and we're working on making it update as often as we have
successful nightly builds. When something doesn't work on the canary, I can
just fall back to my Beta Google Chrome," says Mark Larson, from the Chrome team.
Now that Google Chrome synchronizes bookmarks, settings and it will also synchronize extensions, passwords, browser history, it doesn't even matter that the two builds use separate profiles. Unfortunately, you can't make Chrome Canary your default browser. Google's explanation that it's "a secondary installation of Google Chrome" doesn't make any sense.
Lee Mathews from DownloadSquad thinks that having four flavors of Chrome is "Vista-esque". Google probably noticed that there are many people who install random Chromium builds and decided to offer a channel that updates even faster than once a week, while allowing users to install a stable version of Chrome, just in case the "bleeding edge" builds have major bugs or they're unstable.
Google Chrome Canary builds - Windows-only, for now.
{ Thanks, Tim. }