SQL is a query language to operate on sets.
It is more or less standardized, and used by almost all relational database management systems: 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.
SQL: a language for talking to the database. It lets you select data, mutable and create database objects (like tables,
views, etc.), change database settings.
PL-SQL: a procedural programming language (with embedded SQL)
T-SQL: (procedural) extensions for SQL used by SQL Serve