An SDK – otherwise known as a software development kit or devkit – is a collection of software programs for the purpose of creating applications across web and/or mobile platforms for a particular operating system.
The SDK will typically contain a wide range of tools, libraries, documentation, code samples, processes, and guides that allow developers to build an app for a platform.
For example: both the Android and iOS platforms have separate SDKs that serve as a foundation for mobile app developers.
Say, for example, you want to build a specific functionality for your app which doesn’t exist yet — such as a way to share some text from your app to Twitter.
Instead of building the code from scratch, your developer might start by checking out the Twitter Android SDK to find code that would work on an Android device. Your developer is now assured that this code will work well with Twitter since it comes from the company. And it helps him be more efficient by not needing to build it from the ground up.