Markdown is a markup language that’s used by StackOverflow and many other sites that I absolutely love. It converts **bold** into bold, `code` into
code, etc. You can check out the official Markdown Syntax for more details and examples.
For those unsure what “problem” this addresses, it’s the problem of entering formatted text on the web. If you’re a blogger, for example, that has to insert images, bold text, headers, inline code, etc, you have many options. All these options ultimately generate HTML code. So, of course, you can just write HTML code yourself, but I find this to be (1) error-prone and (2) less readable during the writing/editing process. The benefit of writing HTML from scratch is that you only use the keyboard (no mouse) and have full control of the final result.
One step up from writing HTML code from scratch is writing in a slightly higher-level markup language like Markdown. It’s more readable, requires less typing, and still has the benefit of allowing you to keep your hands on the keyboard, and maintain good control of the look and feel of the final HTML.
StackOverflow (among many others) is a site that has embraced Markdown. That’s what sold it for me. The simplicity and power of a StackOverflow textbox is undeniable (at least to me).