Microsoft has been historically very strong in providing programming tools and also creating frameworks for developers, so it could as no surprise that Microsoft has heavily invested in building quantum tools to aid developers in their quest of building quantum circuits especially with the language Q#.
The most recognizable element of the QDK is Q#, a high level programming language designed for quantum programs
There are lots of open source tools, not just Q# that come with the Microsoft QDK. For example there are tools for quantum chemistry. Simulating problems in chemistry and material science remains perhaps the most evocative application of quantum computing and would researchers to probe chemical reaction mechanisms that were beyond the classical ability to measure or simulate.
Even learning about Quantum computing, there are tools to help programmers understand the Microsoft stack. The Quantum Katas and Tutorials are part of the QDK and provide quantum examples of Q# code and tutorials.
To learn more about the QDK go here.