It is important to look at it as a separate entity, yet it must be able to fit into any conceivable medium. It must illustrate concrete elements such as your product or service, as well as less tangible elements like your philosophy and personality. Although it is a temptation to use an existing image from a web site, brochure, etc., eventually you will run into limitations.
A logo designed by Cranking Graphics will work in any situation from engraving to animation.