SQL stands for Structured Query Language. It is the most common tool used to create, delete, manipulate and manage data in a relational database (often referred to as a "SQL database").
SQL is commonly pronounced "sequel." Its most popular variants are Microsoft SQL, MySQL, PostgreSQL, and SQLite - a version of SQL which is commonly used for prototyping. It introduced the concept of accessing many records with one single command, using SQL Queries.
However, SQL code can differ between the different variants. For example, Microsoft SQL is programmed with T-SQL, which sometimes has a different syntax.