From:https://github.com/mojombo/jekyll/wiki/YAML-Front-Matter

Any files that contain a YAML front matter block will be processed by Jekyll as special files. The front matter must be the first thing in the file and takes the form of:

---
layout: post
title: Blogging Like a Hacker
---

Between the triple-dashed lines, you can set predefined variables (see below for a reference) or custom data of your own.

IMPORTANT! (Especially for Windows users)

When you use UTF-8 encoding for your file, make it clear that no BOM header chars in your file. Or everything will blow up!

Predefined Global Variables

Variable Description
layout If set, this specifies the layout file to use. Use the layout file name without file extension. Layout files must be placed in the _layouts directory.
permalink If you need your processed URLs to be something other than the default /year/month/day/title.html then you can set this variable and it will be used as the final URL.
published Set to false if you don’t want a post to show up when the site is generated.
category/categories Instead of placing posts inside of folders, you can specify one or more categories that the post belongs to. When the site is generated the post will act as though it had been set with these categories normally.
Categories can be specified as a YAML list.
tags Similar to categories, 1 or multiple tags can be added to a post.

Custom Variables

Any variables in the front matter that are not predefined are mixed into the data that is sent to the Liquid templating engine during the conversion. For instance, if you set a title, you can use that in your layout to set the page title:

<title>Jekyll YAML Front Matter</title>

Predefined Variables for Post Front-Matter

These are available out-of-the-box to be used in the front-matter for a post.

Variable Description
date A date here overrides the date from the name of the post. This can be used to ensure correct sorting of posts.