Steve Friedl, Microsoft MVP, developed an awesome and highly detailed article on how SQL Injection attacks work. DBAs and System Administrators need to be on the latest and greatest SPs and security updates for SQL-Server and other RDBMS's. Excellent Article: SQL Injection Attacks by Examplehttp://www.unixwiz.net/techtips/sql-injection.html
SQL Injection is caused by unverified/unsanitized user input, and its main idea is to convince the application to run SQL code that it was not intended to run. If the application is creating SQL strings natively, i.e. on the fly, and then running them, it's straightforward to create some real surprises. There have been other papers on SQL injection, including some that are much more detailed, but this one shows the rationale of discovery as much as the process of exploitation.