Transact-SQL (T-SQL) is an extension of the SQL (Structured Query Language) used primarily with Microsoft SQL Server and Sybase. It enhances standard SQL by adding procedural programming features, allowing users to write complex queries, manage data, and control transactions more effectively. T-SQL includes features like variables, loops, and conditional statements, which enable developers to create more dynamic and efficient database applications. It is widely used for tasks such as data retrieval, manipulation, and database administration, making it a crucial tool for database professionals working with relational databases.