A Software Development Kit (SDK or "devkit") is typically a set of development tools that allows a software engineer to create applications for a certain software package, software framework, hardware platform, computer system, video game console, operating system, or similar (find more from Wikipedia)...
There is a separate section in Forum Nokia for the SDKs for different developement platforms. You may find this page very usefull: Nokia Tools and SDKs mobile applications development.
{Please add the related links here}