SQL (Structured Query Language)
Structured Query Language (SQL) is a widely used, standardized computer language designed to create, modify and query relational databases. Most popular database systems (e.g. MySQL, Oracle, PostgreSQL, SQLite, Microsoft SQL Server, ...) use SQL. Most programming languages provide convenient ways to send SQL commands to a database and to receive the results of the queries and to process them in programs (e.g. to create database driven websites like this wiki).
For more information see the corresponding Wikipedia Entry.