GUI (Graphical User Interface)
Bugs that affect the presentation, look or layout of pages, images and form elements.
Examples: misalignment, broken images, wrong sized graphic elements, inconsistent colors on a link, button or menu, etc.
Bugs that produce an unexpected/illogical application behavior where the end result differs from the expected result.
Examples: search returns the wrong results, clicking on a link takes you to page X instead of the intended page Y, etc.
Bugs that produce error messages or malfunctions of the application.