We don't consider a logo as just a mark - it reflects a business's commercial brand through the use of shape, fonts, colour, and / or images. A logo is for inspiring trust, recognition and admiration for a company or product and it is our job as designers to create an identity that will do its job. A logo must be describable, must be memorable, must be effective without colour,  must be scalable i.e. effective when just an inch in size.