Jekyll is a static site generator, an open-source tool for creating simple yet powerful websites of all shapes and sizes. Here is a little quote from the official website:

It takes a template directory containing raw text files in various formats, runs it through Markdown (or Textile) and Liquid converters, and spits out a complete, ready-to-publish static website suitable for serving with your favorite web server. Jekyll also happens to be the engine behind GitHub Pages, which means you can use Jekyll to host your project’s page, blog, or website from GitHub’s servers for free. - jekyllrb.com

Check out the Jekyll docs for more info on how to get the most out of Jekyll. File all bugs/feature requests at Jekyll’s GitHub repo.