SQL stands for Structured Query Language. It is an ANSI/ISO standard language for querying and otherwise working with relational databases. Although there is a standard for SQL, database vendors typically implement slightly different variants of the language (e.g., in terms of the data tpes supported). In addition, vendors often add procedural extensions to the language (e.g., PL/SQL for Oracle or Transact-SQL for SQL server).
Essentially, SQL is a language implementing the relational algebra, adding features such as Data Types, tranasctions, support for indexes, and other features needed by a concrete database management system