In life...
We witness the world with our five (or perhaps six) senses and for the sake of this statement, say that everything falls into a broad category of objects and concepts. When these objects and concepts are not on hand to directly show another person, certain representations are necessary.
At least on a conscious level, our primary sense is argued to be sight, such that images (photography and illustration) and symbols (the written word) have been developed to represent objects and concepts. In graphic design, these images and words are coldly known as "content" and graphic design itself is its delivery vehicle - the point of contact with the end user's eyes and then mind.
Good graphic design delivers content to the viewer using techniques that capitalize on perceptual principles, even complimenting the content, without totally becoming the content itself. When this does occur, what one sees is no longer graphic design, but art. Art is just as wonderful an endeavor, but none-the-less, a different one.
Design is to order. Graphic design is to visually order information.
In business...
Organizations with agendas are the most prolific users of graphic design. For this reason, a graphic designer must devote a good part of his intellectual resources towards understanding business goals, marketing, branding and any other perceptual connections a company may have or seek with its audience. In order for his efforts to align with an organization's position, a graphic designer - depending on the scale of the organization - must be ready to enter and assist at any of the following phases of the brand strategy process:
I. Brand Audit
II. Synthesis and Strategy Development
III. Brand Design
IV. implementation
V. Cultivation and Management