When a developer can’t find a native solution on the force.com platform, they use the Web Services API to create behaviors that fall outside the platform’s constraints. The Web Services API opens up data and capabilities of ANY app running on the platform. Essentially apps that utilize this API are called “composite” apps, as opposed to “native” apps.
For example, last month’s Silicon Valley Salesforce.com User Group was hosted by DataSea, which is a new startup that has developed a “composite” app. DataSea uses the Web Services API to pull down relational SFDC data, and stores it a neural network for advanced filtering. This capability is not available on the Force.com platform, therefore, the Web Services API makes it possible.
To access Salesforce.com’s latest docs on the Web Services API, go here.