Web API Basics

I’m trying to learn how to use web API’s such as these:
[LIST]
[]http://del.icio.us/help/api/
[
]http://www.flickr.com/services/api/
[*]http://apidoc.digg.com/
[/LIST]
They always list out URL’s needed to grab images or info from those sites. But they don’t tell you what code you need to write to use those URL’s. Does anyone have a link to a good tutorial on general API’s? I’m completely new to this.