Markdown, Taskpaper and Org mode
Markdown is a format that takes a sub set of HTML, that are commonly used when writing and makes it really easy to write. Like headers, links, images, making text bold, cursive etc. Everything in markdown has a 1:1 mapping with HTML and when markdown don't support it you can use regular HTML.
Taskpaper is a format for writing todo lists in plain texts. It has concepts like projects, tasks, text and tags.
Org mode is a insane space ship that you can use for a long of different stuff. The basics of org mode is a bunch of formatting and meta data constructs. Like how ot make outlines, formatting text, linking things together and adding meta-data either to files or notes in a outside. And then you have ways of embedding code. Either as code you run from the document or HTML that is embedded in it. Unlike markdown there is no 1:1 with HTML. And it can do most of what markdown can, but not all of it while at the same time having similar features as taskapper. And there are also a lot awesome features in emacs either built in or as packages making it a insane powertool.