It may be something as simple as an application programming interface in the form of some files to interface to a particular programming language or include sophisticated hardware to communicate with a certain embedded system. Common tools include debugging aids and other utilities often presented in an IDE. SDKs also frequently include sample code and supporting technical notes or other supporting documentation to help clarify points from the primary reference material.
A software engineer typically receives the SDK from the target system developer. Often the SDK can be downloaded directly via the Internet. Many SDKs are provided for free to encourage developers to use the system or language. Sometimes this is used as a marketing tool. For example, Foo Products might provide the Widget SDK for free to encourage people to use it. In turn, more people will be encouraged to buy more of their widgets since they can program them for free.
SDKs may have attached licenses that make them unsuitable for building software intended to be developed under an incompatible license. For example, a proprietary SDK will likely be incompatible with free software development, while a GPL licensed SDK will likely be incompatible with proprietary software development. LGPL SDKs are typically safe for proprietary development.
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.