T-SQL stands for Transact-SQL. It is a Microsoft extension to the SQL language and it is the native language of SQL Server.
T-SQL is a cross between SQL and Visual Basic for Application (VBA). It can retrieve, edit, add and delete data from tables in an SQL Server database as well as run functions like other programming languages. It can pass parameters, perform calculations, execute library function as well as custom functions. This combination of the data and programming capabilities in T-SQL makes SQL Server very powerful.