There are really only two remaining eBook formats, of the numerous types that were floating around some years ago.
MOBI: The first and foremost, in terms of commercial sales, are the Kindle format(s) for Amazon. Those are :mobi, KPF and (for previewing only) .azk files. (updated 1-28-2021) Colloquially, these are called "mobi" by most people in the business.
ePUB is used by Apple, Adobe Digital Editions, Nook, Google Editions, Diesel, Kobo, and most Droid-based readers is ePUB. ePUB allows greater design flexibility than mobi, because it uses a more advanced level of HTML.
Two other formats, Palm and .LIT, (which was Microsoft's attempt to gain entry into the eBook marketplace), are not supported any longer in any significant numbers, and so we do not normally include conversion to those formats as part of our conversion quotes. Palm used a lot of formats that appear as PRC or PDB files on regular computers.
Don't be fooled by those companies offering "conversion to all 6 formats!" or that type of talk. In today's marketplace, you need two formats, and two only. That's MOBI and ePUB. The first one covers Amazon. The second covers literally every ebook retailer still standing, in 2021. Companies offering that type of "too many format" options are likely auto-converting their ebooks with a free program that you can download yourself on the Internet. The "all those formats" approach should warn you OFF using any formatter that promotes this!