Basic Plain Text (BPT) is Markdown supplemented by text-edition-specific extensions.
Markdown is not a single and strictly-defined entity. Several flavours of Markdown exist, such as CommonMark, Scholarly MarkDown, GitHub Flavored MarkDown, and of course the original Markdown. The difference is in the extensions.
Markdown is simple to use. Anyone who has ever written an email has probably written in Markdown without knowing it. Remember those asterisks people use to **stress** something? That’s Markdown. You can learn it in 60 seconds.
BPT uses Markdown as a semantic format, i.e. to mark meaning rather than representation. For example, in BPT we use underscores to _emphasise_ text and asterisks to mark **important** text. The decision about how to render these marks is taken elsewhere, for example in the script that creates a PDF from BPT files.
This is similar to how HTML uses <em> to tag emphasized text instead of <i>, and <strong>to define important text instead of <b>. Indeed, Markdown was conceived in 2004 by John Gruber as a “text-to-HTML conversion tool for web writers.” For this reason, any HTML tag is valid Markdown.
There is no need to repeat here what has been explained clearly elsewhere.
See this cheatsheet for an excellent overview of basic Markdown syntax.
For some hands-on experience, follow this lesson. Highly recommended!