SQL injection is when it is possible through the regular web application interface, run database query that application creator does want to be possible to execute. We share with you an excellent resource about SQL Injections, NetSPI SQL Injection Wiki!
SQL Injection Resources
Modern web frameworks are bulletproof against SQL Injection out of the box. Problem is when developers override web framework API. There is a ton of SQL Injection resources, but NetSPI SQL Injection Wiki is one of the best organized that we so far have found on the Internet.