SQL (Structured Query Language) is a standard interactive and programming language for getting information from and updating a database. Although SQL is both an ANSI and an ISO standard, many database products support SQL with proprietary extensions to the standard language. Queries take the form of a command language that lets you select, insert, update, find out the location of data, and so forth. There is also a programming interface.
You can find out more about SQL at the SQL Standards Home Page.
John Barkley's section on Security with SQL is available at the National Institute of Standards and Technology (NIST) Web site.
As an introduction, we've read and recommend Allen G. Taylor's SQL for Dummies, Third Edition.
For Web site owners or others who use or are thinking about Microsoft's SQL Server, we recommend Ron Soukup and Kalen Delaney's Inside SQL Server 7.0.