What is SQL, PL/SQL, T-SQL and difference between them?

• SQL stands for Structured Query Language is a query language to operate on sets. It is used by almost all relational database management systems like SQL Server, Oracle, MySQL, PostgreSQL, DB2, Informix, etc.
• PL/SQL is a proprietary procedural language used by Oracle
• PL/pgSQL is a procedural language used by PostgreSQL
• TSQL is a proprietary procedural language used by Microsoft in SQL Server.

Procedural languages are designed to extend SQL's abilities while being able to integrate well with SQL. Several features such as local variables and string/data processing are added. These features make the language Turing-complete.

They are also used to write stored procedures is a pieces of code residing on the server to manage complex business rules that are hard or impossible to manage with pure set-based operations.