Technically, Markdown is a "text-to-HTML conversion tool for web writers," but - as it is implemented today - there is little need for the average user to consider anything technical beyond personal use.

Markdown Demonstration Outline

Current Supporting Platforms

...almost.

...almost.

...and a lot more.

References

Markdown throwdown: what happens when FOSS software gets corporate backing? | Ars Technica

Daring Fireball: Markdown