What's SQL Server SSDT?

SQL Server Data Tools (SSDT) is a toolset that allows professional database and application developers to carry out all their database design work for SQL Server and SQL Azure within Visual Studio. SSDT provides a rich SQL Server development experience with first class language services and VS integration, as well as declarative, model-based tools that can be utilized for both online and offline development.

This's a good presentation to have idea about it:
https://www.youtube.com/watch?v=uPll3dMxTXU