The 'Rule Of Thirds' is a compositional tool used to maximise the interest of a painting, photograph, render, or just about any kind of 2d image.
I feel almost guilty that I've not taken it upon myself to learn this important composition rule, but, as of late, I've started to build confidence in my 2D painting and drawing abilities, and have decided its time I learned how to not only draw accurately, but to be able to create compositions well.
So, basically, the rule of thirds states that pictures should be broken up into 9 squares (by dividing an image into thirds on both the x and y axis). These lines serve as a placement guide on the image and the point is that you place interesting items of the scene on these lines. Better still, place interesting things at the points where the lines meet.
Image taken from Wikipedia
Such a simple set of rules, but makes perfect sense, and breaks up the monotony of having front on composition all the time. I shall be applying this technique to many further 2d works!