Like anything, it's just dedication. In art there really isn't talent, anyone can do it, but you have to have the drive to do it. That's the difference between a great artist and someone who just draws. I'm not talking about art nouveau either, where the person splashes some paint on a canvas and calls it art. I'm talking about true representation, impressionist, academic classical, realist, and victorian artists. Akin to the stuff you'd see on this site
Art Renewal Center. The great artists will draw with every waking moment they have, and they will train their eye to see shapes and colors more effectively. Someone that wants to casually learn art or painting will never excel at it. It's one of those things you have to do at 100%.
Art is definitely something that can be taught. For some it's just harder to grasp the concepts than others, but everyone could do it if they wanted to. That's the beautiful thing about it. I know artists who were some of the worst in their class in high school when it came to technical skill, but by the time they came out of college, they excelled at their craft. It's all in the drive.