TXT Plain Text File
TXT is unadorned plain text: characters, line breaks, nothing else. There is no bold, no styles, no images, and no structure beyond what the words themselves imply, which is precisely the point of the format.
The one genuine complication is character encoding. A .txt file does not reliably announce whether it is UTF-8, UTF-16, or a legacy code page, so the same bytes can render as different characters depending on what the reader assumes. UTF-8 is the sane modern default.
What TXT gives up in expressiveness it returns in permanence. Every operating system, editor, and programming language reads it, it survives decades without special tooling, and it is the most reliable substrate there is for text you intend to keep.[1][2][3]