It's a bit of a broad spectrum, being honest. And people have asked me this before, and I never know what to say exactly, I just tell them to try it. But, it's basically a cross between photo manipulation and creating effects. Or creating surreal concepts through the use of a photo editing program. But there's different levels of graphic art and a load of different topics. Such as typography, digital painting etc.
But it's most things which fall under another broad category such as pixel art and 3D models.Graphic art is very much mixed media.