Text editors are used for creating and editing files consisting of ascii text only.  That is, no formatting codes are being hidden, the way they are in word processors.  There are several popular editors and choosing one is mostly a matter of personal taste.  My personal favorite, for example, is Programmer's File Editor (PFE) on Windows, and vi on Unix.  Many people like Emacs and XEmacs for Unix and Emacs is even available for Windows.