API stands for application programming interface. It's basically just a set of rules to define how certain kinds of software work together. People can use Google's API to develop applications that use Google's proprietary technology. So you could develop an application that automatically grabs content from a high ranking site for a keyword search, or maybe your cell phone can use Google Maps data to find restaurants or movie theaters. There are subscription software services that use Google's API to spy on Adwords data. So an API acts as an exposed interface to a proprietary technology. Hope this helps.