This was the first real question I remember being asked in my first year of college. It has stuck with me, evolved, and become a core part of my work ethic since then. When you understand what graphic design is, you can then understand what a graphic designer is, make smarter choices when you seek the services of one, and hopefully understand that “good” graphic designers are not necessarily a “dime a dozen”.
I associate the core meaning of design with “plan”, both as a thing and an action—with clear intents, goals, methods, etc.
A graphic is a visual image on a surface as a method of communication. It’s important to remember that it’s not limited to brochures and documents—It’s walls, signs, tattoos… Any and all are composed of any number of elements such as images and text.
Graphic design therefore, is a visual image created in accordance to the plan of what is to be communicated. This need for a plan separates the crossover of art and design to an extent. Graphic design can be an art because the visual form of the design might include considerations of fine art as appropriate. This does not always translate the other way, in that art without a plan for clear communication isn’t necessarily graphic design.
To put it another way: A successful art form can be completely subjective in it’s meaning. Successful design should be clear and objective in it’s communication, and not just a “pretty picture”.