[This API is a dupe of the Salesforce SOAP API and the Salesforce Force REST API. This profile is maintained for reference purposes only.]
Saleforces's Metadata API is used to assit developers with retrieving, deploying, creating, updating, or deleting customized information. This can be such things as custom object definitions and page layouts for organizations. The Salesforce Metadata API is meant to be used for management of customizations and building tools to assist in managing the metadata mode.
Salesforce has introduced a new Apex Metadata API for Force.com to help developers build self-configuring applications, upgrade scripts, and custom set-up user interfaces. The API allows applications to enable capabilities such as retrieve metadata synchronously and set up a callback on completion.