The most common type of API though has to be those aimed at web development. Tools for web development are often published freely (though retaining proprietary rights to the code), with the overall intent of facilitating the consturction of an open architecture for web content and data between different applications and online communities.
Web APIs are typically a defined set of HyperText Transfer Protocol request messages, coupled with a definition of the structure for response messages. Often coded in Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format, they allow the combination of multiple services into new online applications commonly known as 'mash-ups'.
The ability to openly use Web APIs has allowed the creation of a variety of now commonly used web services. Perhaps the best example is the ability to embed content from one web page onto another web page through relatively simple HTML code - for example, posting a YouTube video on a web page rather than simply posting a link to YouTube. Similarly, an API lies behind the ability to upload a video on Flickr and post it on Facebook or MySpace, as well as the ability to stream live feeds from social media networks such as Twitter on other sites.