Markup language are text-encoding systems that use tags or symbols to determine how text should be displayed when processed by the computer. Markdown, just like HTML, is a markup language. But, HTML syntax is difficult to write and read by humans. Meanwhile, you can read Markdown as easily as regular text.
Back in the early 2000s, writing for the web was confusing. If you wanted to talk the talk, you had to learn some HTML. Then came John Gruber, an early blogger and UI designer. He was heavily involved in the tech scene, primarily through his blog Daring Fireball. While he loved to write and publish online, he didn't like the idea of formatting everything using HTML.
Inspired by the aesthetics of plain text email communication, Gruber started working on Markdown in the early 2000s. His overarching goal was to pluck the process of creating digital documents from programmers and give it back to writers.
Aaron Swartz -- a programmer and a political activist that contributed to many Internet milestones, including the development of RSS 1.0, Creative Commons and Reddit -- like Gruber, wanted a user-friendly way of formatting text that'd be more natural and intuitive than using HTML tags. At some point, Gruber and Swartz communicated and the latter became Markdown's "sole beta tester." Swartz provided feedback on the syntax.
Finally, Markdown 1.0.1 was released on 17 Dec 2004.
The overriding design goal for Markdown's formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text. The single biggest source of inspiration for Markdown's syntax is the format of plain text email.