.ePub, .ePub2, ePub3 (Electronic PUBlishing from Adobe) - the most common in the world. In fact, this is a website packed in an archive. It is supported by almost all readers.
.FB2 (FictionBook) - format (standard), developed by Dmitry Gribov. Ideal for creating e-books, perfectly structuring books, takes a small amount, well-converted into other formats. In fact, this is an XML file. But in the world it is almost not known and therefore a little what Western reader supports it. It is supported by almost all Russian, Ukrainian and Chinese readers.
.MOBI (Amazon Kindle) - format for the Amazon Kindle reader. Other readers do not support it. The converter will help here.
.PDF (Portable Document Format from Adobe) - a common format for electronic documents: book instructions. Typically, quality, it is convenient to read, on the computer, it contains good formatting, inserted pictures, graphics. Suffice it to say that books and newspapers are printed in this format. For readers it does not fit very well, does not fit into the window, its division into pages ... However, this format is supported almost everywhere.
.DjVu (deja vu) - collected and compressed scanned documents, by the way, not very high resolution. Readers rarely support. The format is disgusting, but it's easiest to create a book, because some books, especially dictionaries with many languages, old books ... are more often found only in this format. I recommend only if there are no other options.
.RTF (Rich Text Format) - a universal format for text documents. You can edit or create it yourself. Readers rarely support. You are recommended to convert to ePub.
.DOC - Microsoft Office format. You can edit or create it yourself. Not all readers support. Here somehow not for that they are made. You are recommended to convert to ePub.
.TXT - just a text file, without formatting, the book will open with any program. You can edit or create it yourself.
.JAVA - for reading on a mobile phone. Support almost all phones.
This is not all formats, there are many more. But this is basic, the rest is specific.