Kobo is an eBook store with their own eReaders, like Amazon Kindle or Barnes and Noble Nook. You can use Kobo with one of their devices, or you can use an app on your phone or tablet. Like Amazon and Barnes and Noble though, they want you to only read the books through their apps or devices.

My opinion on Kobo is that it's pretty good. They do daily deals where they massively discount books for 24 hours. I've gotten some good deals out of it.