Etext also known as e-text and eText is a generic term for all forms to text that are in electronic form.
eBooks are a subset of etext which is a term for any form that electronic information using characters. It includes all forms of HTML which can be termed tagged etext or it can be a simple TXT file with no formatting at all, sometimes referred to as an ASCII file.
Etext also includes email, enewspapers, emagazines, documents, and eTextbooks. These may be in a format that is not directly readable without an app but are called etext since they are the electronic form of a physical document. Many PDF files and other eBook formats fall into this category.
The names often begin with an e to indicate the electronic form. The e may or may not be followed with a hyphen. Most of these usages have dropped the hyphen.
Some of these texts sources may never be expected to be released in electronic form but are instead the the storage format used by a word processor. They are only intended to be a storage media for the source of a book or other document. These are usually stored in a binary format and are not defined in the term etext. Some formats, notably RTF are technically text files but are not intended for humans to read directly thus these too are not considered etext.