Listen to limdis, he knows about what he's talking.
On top of SQL, there's quite a bit more you'll prolly wanna get down. At it's core, hacking is about knowing multiple technologies and how they can be used together to accomplish a goal that was otherwise unintended. It's "surfing" the boarders or boundries of two or more technologies.
Also, there are different types of hacking, that require knowledge in different areas. It seems that you're specifically focused on web hacking, which is just one of the different fields.
When it comes to web hacking, you'll want a pretty solid understanding of:
PHP (and other scripting languages)
Enough experence in programming in general (This will help you to infer from context how the application was written)
Apache or IIS (and to be really good, both)
*nix based OSs and even Windows (server 2003/2008 specifically, with IIS role)
Knowing just a handful of these technologies will limit you to only being able to "hack" those sites that use exactly those technologies.
On top of that, there's an immeasurable element to understanding how all those components interoperate.
To answer your question VERY simplistically, SQL injection is entering a SQL command into any input field that takes user input (an is unsanitized, ie doesn't filter characters like single quote, double quote, colon, semicolon etc) to "escape" the written code, invoking an unintended action on the part of the web developer.
-- Tue Feb 19, 2013 8:49 pm --
BTW, let my stipulate that I'm ONLY advocating doing this stuff hypothetically, or on your personal website (running on your localhost or a VM, speaking of which, get VirtualBox and a linux distro)
This will help you in learing all these technologies, like Apache and well...IIS will be a bit harder, but luckely, IIS is a build in component in Windows 7 (and XP iirc)
-- Tue Feb 19, 2013 8:54 pm --
Build a website that uses all, or the majority of the tags (HTML 4, you don't REALLY have to go all out and use HTML 5)
After that, setup a MySQL DB and try to impliment that into your website. Doing all this will give you the added bonus of learning a littel linux. Personally, I prefer Debian based distros like Ubuntu (for ease of use) and BackTrack (just a quick and ready to go distro with a TON of tools preinstalled)