Jekyll is a very simple tool that helps create static sites. The sites which do not require headaches of server, security, databases etc.
It is a very popular tool. And has a large online community. It has a lot of plugins etc and can help you create your own website in minutes.
Jekyll is blog aware, it can be used to build any type of site as long as its static by nature. Github Pages are highly built on Jekyll.
Even this website is also built on Jekyll. The best part of Jekyll is that you can build templates that are flexible. You can include parts of files, generate feeds, and automate a lot of stuff using plugins.