Technically, official Kobo books are KEPUB, which is their own proprietary version of EPUB they use for their store, but they can read EPUB and other formats just fine. And if they don’t, Calibre solves that problem.

(Converting books to KEPUB is sometimes worth it, especially if they have illustrations mixed in with the text, because then you are able to do things more easily like zoom in to the image on the reader.)