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Originally Posted by stasiana
What is actually the difference between epub, epub3 and kepub?
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This is a simplified explaination
It's epub2 as epub1 was called OEPS or OEBPS (Open eBook Publication Structure) and dates to about 1998.
The epub3 has many different options and the epub2 ToC is optional for "fully" epub3 based apps, but needed for many ereaders and apps. The native epub has a different system contents file. Also epub3 has many optional features, some of which are multimedia or interactive and thus not needed for "real" ebooks that are simply text and images. Additionally epub3 has a fixed layout option to do paper book replication, but differently to PDF (which is deliberately EXACTLY what a print on paper would be and used to generate paper print).
Likely if you "Polish" and "upgrade" internal you get an epub3 with no fallback epub2 ToC. This can be automatically added in Calibre Editor.
The epub3 from Gutenberg have no "fancy" epub3 features and have also the fallback epub2 ToC file. Both sorts of epub are renamed zip files. If you rename to zip you can extract all the files! Handy to get the images without Calibre.
In theory, if you knew what you were doing you could use a text editor and repack to zip and rename to epub. It's better to use Sigil or Calibre.
The kepub is Kobo's proprietary extension to epub2. It has extra <span> stuff </span> in the HTML files. The epub on a Kobo uses Adobe renderer, Adobe Digital Editions (abbreviated ADE). The kepub files use Kobo's own renderer which allows zooming images and popup footnotes but does the text layout differently. Some people prefer kepub and some prefer epub2.
An epub2 and epub3 can look almost identical, and pretty much will be if the ebook was automatically created from a properly styled docx file and is not fixed layout epub3.
The epub3 has better support for semantic based HTML that isn't recognised by epub2, but the onscreen appearance may be identical. Epub3 is better for textbooks with support for equations and javascript.