The logo, or brand is not just a graphic but it is the embodiment of an organization - of its area of work, it's ethos, it's core values and so on. Designing a logo is commonly believed to be one of the most important areas in graphic design and thus taken as the most difficult area to perfect.
A good logo is unique, and not subject to confusion with other logos among customers, is functional and can be used in many different contexts while retaining its integrity. A good logo should remain effective reproduced small or large, can work in "full-color", but also in two color presentation (black and white), spot color or halftone