Here ill be trying to answer the question of what is graphic design?
A good a start as any would be the wikipedia definition
"Graphic design is the process of communicating visually using text and images to present information. Graphic design practice embraces a range of cognitive skills, aesthetics and crafts, including typography, visual arts and page layout. Like other forms of design, graphic design often refers to both the process (designing) by which the communication is created and the products (designs) which are generated."
So what its saying is its getting a messege across visually, but its not just the final product that is graphic design, but the process leading up to that final product that is graphic design also.
theres a short video illustrating some of the key points of the process.
But graphic design is all around us, the world we live in has all been designed, by various different types of designer, graphic designer is in the broad field of anything visual, as a architech is a building designer and designs buildings in which to live in, a graphic designer designs graphics/imagery/visual information that is useful to the viewer/target audience.
Heres some very early graphic design from the book of kells, a book illustrated by celtic monks in 800ad of the new testament. The monks wanted to get across the glory of god within the pages and thus designed it to that effect, graphic design over a thousand years before the term was invented.
the gutenburg press (above) was the first printing press making mass multiple prints possible. whoever pieced together and carved the plates for it was a graphic designer.
Mondrian is credited with being one of the founding fathers of the "grid" system, commonplace in design today, in which all the elements on the page/surface are arranged according to each other on some kind of grid, works such as the one above by mondrian inspired this.
The term graphic design was first coined by William Addison Dwiggins, an American book designer in the early 20th century.
Graphic design moves through different styles and eras much like fine art does, as it tries to capture the publics need at the time.
bauhaus really founded modern graphic design, and was one of the first to teach more modern design principles, they're one of the most important schools in graphic design history. Outlaying may of the rules we still adhere to today.
constructivism came out of russia in the 1920's and was heavily linked to a political ideal. Its another example of a era of design.
this style of 1960's design is very recognisable, encompassing the free spirited nature of the era.
contempory design is a lot more open to anything, but it still has its styles, such as the grunge style above. now that everyone has access to computers, anyone with rudimentary knowledge can hash together some design, this is where it comes back to process, truely great design has great process behind it.
There are lots of rules to graphic design, and a saying that "you have to know the rules to break them", ill leave you with david carson's take on this.