A text editor is a program which edits files in plain text format, as compared to binary format. Using a non-text based word processing program (e.g. using Microsoft Word to edit PHP scripts) can cause major problems in your code. This is because non-text based word processing programs insert extra formatting into text files, and can corrupt the files when they need to be interpreted by the interpreter. An editor like Notepad does not insert any extra formatting.

Edit WordPress Files with a text only editor.

Some examples of file formats which need to be edited as plain text:

Some examples of text editor programs:

Some examples of non-plain text formats that require special software for editing:

  • Microsoft Word documents
  • Microsoft Excel spreadsheets
  • Images, such as JPEG, PNG, or GIF

Some examples of software which can edit text, but which are NOT regarded as basic text editors and NOT recommended for use on WordPress files:

  • Microsoft Word
  • Microsoft Works
  • Microsoft Excel
  • Adobe Photoshop
  • Adobe Illustrator
  • Adobe Dreamweaver

Reference http://codex.wordpress.org/Glossary