Lets start at the top and work down. Art directors, or Creative Directors are responsible for a creative team that may design work for magazines, television, advertising graphics, websites, or on packaging. A creative team can consist of layout artists, graphic designers, photographers, copywriters, and menial staff to do the work. An Art directors job is to make sure that each of these workers do not slack off down the pub and complete their work to a deadline and to the client's needs. Art directors also make major decisicions along the lines of should the background be slate grey or cobalt blue, issuing dictates and changing their mind several days after a deadline has passed - leaving co-workers resolutely glum about their position in the grand scheme of things. Art directors will inevitably have come from some kind of marketing or sales background and need no prior graphic design knowledge or skill.
A Senior Designer is mainly concerned with the visual aspects of a company and will probably have been promoted on the basis that she is fun and a 'great team player' (despite this being far from the case). A Senior Designer will usually insist on having a larger widescreen monitor than the rest of the team which will be decorated variously with fluffy pink bits marketing people send through on a daily basis. A Senior Designer will be involved in the elements of a company's look such as business cards, stationery, packaging design, media advertising graphics, promotional design, and sticking up pictures of topless 'hunks'.
The job of a Graphic Designer is to provide a new and exciting way to express the key information of a company or product through a dynamic image or use of typography. Graphic Designers take the scant information given to them by the client and using the internet to scab some free clip art, fashion their own ripped off logo designs in order to fleece the client for all they are worth.
Layout Artists and Artworkers
The engine room of the design world. These scumbags have been buried away with their dusty macs for decades, remorselessly churning out pages and layouts. Inevitably some clueless muppet will send over a 100 page brochure layed out in microsoft word and it will be the Artworkers thankless task to make it publishable. They will need to recognise a font at 50 yards, be able to colour correct the dreariest of images and take a good bollocking every now and again to keep them on their toes. The Artworker must have the ability to design magazines, design brochures, design flyers, design books and design posters. He harbours murder fantasies.
Illustrators generally speaking will have long greying hair and be influenced by prog rock. Working from home among the dungeon and dragonns figurines and manga comics they will attempt to put their own unique spin on whatever brief they are given. What you will be presented with is a semi clad girl with oversized boobs. You will have waited several weeks for this. You will never learn from previous mistakes.
Web designers create the pages, layout, and graphics for web pages, they will be technically minded to the point of absurdity. They will insist on using c++ coding language to impress other geeks and will beaver away doing whatever it is geeks do for hours on end. Web designers also design and develop the navigation tools of a site which will for design websites involve tiny text that makes your eyes bleed. Web designers are far too clever for their own good and should never be encouraged.