In very simple words, Graphic Designer is the one who designs, creates, edits, manipulates various types of graphics.
These graphics can be Photographs, pictures, drawings, icons, line arts, paintings etc.
Using all these types of graphics, s/he can create flyers, posters, magazines, hoardings, book covers, brochures, product catalogs etc. S/he can also create web banners, buttons, icons, photomerge (collage), photo editing etc.
Product packaging design, User Interface design, Corporate identity profiling are few such more services a graphic designer can offer.
