All about APIs, mashups, and microformats
I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
There's a great article on Digital Web Magazine called APIs and Mashups For The Rest Of Us by Gareth Rushgrove. The article explains what an API is and how websites uses these extensively nowadays. Mashups are described and the use of microformats as an API is explained [see also Can Your Website be Your API?].
I'm already looking forward to part 2 of APIs and Mashups For The Rest Of Us.