This article explains clearly what is an API and how you can implement one for your own website. Discusses SOAP and REST web services and also provides the codes for implementation.
APIs are a great way to extend your application, build a community, excite your users and get in on the Mashup Mania spreading across the web. While there’s plenty out there wanting in on the action, there’s a lot of questions about how to actually go about creating an API for a web application. Like everything else technical on the web these days, there are tons of complicated and scary documents out there ready to intimidate the unprepared. In an attempt to get everyone on the bus in one piece, we’ve tried to filter through the hard stuff and give an easy to understand starting point for anyone on a quest to API’ify their web service.