Jekyll is a generator that turns plain text and Markdown into HTML. With the simple template syntax Liquid you can build arbitrarily complex websites that only need a web server. This also eliminates a large part of potential security gaps and performance problems. Once the basic framework is created, further data, blogposts, tables etc. can be added only with Markdown and without HTML knowledge.
On the other hand, you have to do without some dynamic features - e.g. a search function - or upgrade them on the client side.
This talk shows how Jekyll can be used to set up websites for different use cases in a flash. It also shows how to retrofit dynamic features like a search function in the browser.