A logo is a simple graphical element whose core function is identification. Logos are often part of corporate design or visual identify. In general, a brand will maintain a single logo (possibly with a small number of variants for different media or 'sub brands').
A logo is a flag, a signature, an escutcheon.
A logo doesn’t sell (directly), it identifies.
A logo is rarely a description of a business.
A logo derives its meaning from the quality of the thing it symbolizes, not the other way around.
A logo is less important than the product it signifies; what it means is more important than what it looks like.