A good design is a marriage of a form and a meaning. It is not just about creating something nice. It is indispensable that the item that the designer is working with (be it a logo or an advertising flyer or a catalog, a business card, a fill-in form) complete the tasks meant for it and do it in a most efficient way.
Design is, in its way, a science, the science of marrying in harmony the aesthetic and the functional. And that in turn means that design entails a certain basis of specific knowledge.
Have a look at some curious and useful facts about design as such and about the items the designer works with, such as logos, corporate style, branding, corporate materials issuance art.