SimpleText evolved from TeachText which was used to distribute Readme documents, which was derived from the Edit application, a simple editing application distributed with the earliest of Macintoshes to demonstrate the use of the Macintosh interface.
In Mac OS X, SimpleText is replaced by TextEdit which reads and writes documents in Rich Text Format as well as ASCII and HTML, and also includes the ability to read SimpleText files (though not edit them). OS X also includes vi and pico as well as other terminal-based text editors).