SimpleText is the native text editor for classic Mac OS. SimpleText allows editing including text formatting (underline, italic, bold, etc), fonts, and sizes. It can be considered similar to Windows' WordPad application. In later versions it also gained additional read only display capabilities for Macintosh-style PICT files, as well as other MacOS built-in formats like Quickdraw GX and QTIF, 3DMF and even QuickTime movies. SimpleText can even, using Apple's PlainTalk speech system, read out text in English and record short snips of voice.
SimpleText evolved from TeachText which was used to distribute Read Me documents, which was derived from the Edit application, a simple editing application distributed with the earliest of Macs to demonstrate the use of the Macintosh interface.
In Mac OS X, SimpleText is replaced by the more powerful TextEdit, which reads and writes documents in Rich Text Format, RTFD (RTF with attachments) format, and Microsoft Word format, as well as ASCII, various Unicode encodings and ISO-Latin, among others 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.
Apple has released the Source Code for a Carbon version of SimpleText in the Panther (10.3) Developer Tools. If the Developer Tools are installed, it can be found at /Developer/Examples/Carbon/SimpleText/.