Chapters and Kobo are kissing cousins--separate companies with Chapters owning 58% of Kobo. The ebook links on the Chapters site take you to Kobobook.com. People need an account there to buy a book from Kobo--the chapters account login will not work.
Kobo will work with PDF and ePUB books purchased from a wide variety of sources. They must either be non-DRM or the standard Adobe DRM (ADE). You can sideload from Calibre without a Kobo account as long as the format is correct.
As an alternative to sideloading, either through Calibre or Adobe, you can sync books bought through Kobobooks. The synced books use a proprietary Kobo DRM. Books added to the reader this way will sync across multiple devices, work with the dictionary (currently WiFi Kobo only), and use the website "I'm Reading" list to do things like keep track of bookmarks across devices.
You may want to download the Kobo user guides.
http://koboereader.com/user-guide