SQL injection is (more or less) the execution of (unauthorized) queries in a SQL server.
There isn't a sure fire method to stop this kind of exploit, but escaping user inputted data will stop the majority of the attacks.
Pay attention, SQL injection may be used in any site using SQL servers, disregarding the type of scripts: Chats, Forums, games or whatever backed by SQL may be attacked.
There is absolutely NO way to be 100% un-hackable, but good protection, (everything from programs at the server to ip shields..)
AND if you learn some basic hacking, you know how to prevent basic and a little more advanced hacking, or how to prevent it 99%
sql injection is the most common way of hacking javascript websites
to make ur site temparyly unvunrable try learning about the way it is down and try to block some of the querries they use to do the exploit like 1=1 OR ' --
sql injection could be used many times, cause it's very usefull. If the login page uses javascript to deny if password isn't right, then that page could be hacked very simple. How to avoid this: remove javascript checking passwords, use for ex php. I'll provide more info if you will need via pm.