what is rich text editor, plain text editor, and markdown mean?

[Zarn]

Rich text editor means that it accepts (and shows) directly coding for bold face, italics, underline, and so on - similar to the formatting that is possible in the RTF file type, Rich Text Format. http://en.wikipedia.org/wiki/Rich_Text_Format

Plain text editor means that it accepts only ASCII symbols (or only Unicode symbols), which would mean that if you were writing HTML and were formatting text to be bold, you'd see the code field itself meaning where bold text started and stopped, rather than actual bold text (which the Rich text editor would show).

Markdown is a quickie form of giving formatting instructions in the text; it's a play on the "markup" part in HTML (HyperText MarkupLanguage). http://en.wikipedia.org/wiki/Markdown