How to learn more about MySQL, Apache and .htaccess?

view story

http://stackoverflow.com – I'm a teenager and I just like computer programming and always like to teach myself various stuff to its greatest extent. For example, after I learned HTML, I jumped on to learn HTML5, and after learning JavaScript, I began learning about closures, DOM, etc. Recently, I was looking at some jobs posted on some popular sites (tumblr, vimeo) and I found out that database-engineering was listed and they were asking for one who knows MySQL taken to its greatest extent. What do they mean by this? I only knew that databases were used to store information and since I know PHP, I only used to query da (HowTos)