(from Entrepreneur.com)

There are basically three kinds of logos. Font-based logos consist primarily of a type treatment. The logos of IBM, Microsoft and Sony, for instance, use type treatments with a twist that makes them distinctive. Then there are logos that literally illustrate what a company does, such as when a house-painting company uses an illustration of a brush in its logo. And finally, there are abstract graphic symbols-such as Nike's swoosh-that become linked to a company's brand.