The name “Application Programming Interface” is very self-descriptive.  When programming an application to interface with a separate application or platform, you will almost invariably use an API.

For example, Amazon.com released its API so that Web site developers could more easily access Amazon’s product information. Using the Amazon API, a third party Web site can post direct links to Amazon products with updated prices and an option to “buy now.”

An API is a software-to-software interface, not a user interface. With APIs, applications talk to each other without any user knowledge or intervention. When you buy movie tickets online and enter your credit card information, the movie ticket Web site uses an API to send your credit card information to a remote application that verifies whether your information is correct. Once payment is confirmed, the remote application sends a response back to the movie ticket Web site saying it’s OK to issue the tickets.

A Software Development Kit is a package of pre-written code that developers can re-use in order to minimize the amount of unique code that they need to develop themselves.  SDKs can help to prevent unnecessary duplication of effort in a development community.

An SDK for an operating system add-on (for instance, QuickTime for Mac OS) may include the add-on software itself, to be used for development purposes, if not necessarily for redistribution. An interesting situation arises here between platforms where it is possible to develop applications that can at least start up on a system configuration without the add-on installed, and use a Gestalt-style run-time environment query to determine if the add-on is present, and ones where the application will simply fail to start. In other words, it is possible to build a single binary that will run on configurations with and without the add-on present, albeit operating with reduced functionality in the latter situation.

Providers of SDKs for specific systems or subsystems may sometimes substitute a more specific term instead of software. For instance, both Microsoft and Apple provide driver development kits (DDK) for developing device drivers.