I was reading This answer and came across this section in it (emphasis mine):
In addition to selling via the big eBook outlets, offer sale of DRM-free (but watermarked, if you like) ebooks in multiple formats on your web site and/or through outlets specializing in DRM-free ebooks. There are a growing number of ebook consumers who are tired of DRM, and their desired rate of consumption, at least in the fiction market, is far higher than the supply serving them.
Now, I get DRM-free, I'm a gamer and DRM is a huge pain in the ass, but I don't understand what formats of DRM-free eBooks there are? Are these just different file types (e.g .pdf, .docx, .txt), or is it something else?