Web development has grown far beyond the typical HTML/CSS/JS code structure. Modern web services like Facebook and Twitter are built with something called an Application Programming Interface. This allows developers to connect into 3rd party services and pull out data to be displayed on another website.
For this post I've collected a number of free online tutorials which delve into API development. These are perfect for anybody new to the scene who wants to learn a bit more about typical web services. Some will require an API key while others may simply return XML/JSON data on command. Either way these articles detail many popular development techniques in an easy-to-understand fashion.