Plain text is simply non-formatted text, meaning that the text has not be altered in any way. In text editors, plain text is used when typing. Formatted text is styled text, such as bolded text or italicized text. Plain text is read as is, without being able to identify any stylized text; paragraphs can be identified since adding a line break is a basic option when working with plain text. For example, when using Notepad, the text you enter is plain text, because their are no options to modify it, unless you save the file as a different type of computer file used to create documents, such as HTML, which will then allow you to use HTML tags to customize the text. The effects added to the text will not be visible because text editors like Notepad do not display formatted text.
Plain text is almost always used in programming software, such as text editors and compilers, as well as some other programs which require direct code input.
In HTML, the <pre> element renders its content as plain text. The <code> element shows its content like plain text, but anything that styles the text is also applied.
For example,
This is <pre> plain text. No <b>formatting</b>, no matter what.
This is bold and italic text which looks like plain text.
Plain text is usually displayed in a monotype (typewriter) font, such as Lucida Console or Courier.