Legendary designer Paul Rand perfectly summarized what a logo is and does: 

A logo is a flag, a signature, an escutcheon.

A logo doesn’t sell (directly), it identifies.

A logo is rarely a description of an entity, product or service.

A logo derives its meaning from the quality of the thing it symbolizes, not the other way around.

A logo is less important than what it signifies; what it means is more important than what it looks like.

A well designed logo will be: distinctive, legible, usable, memorable, universal, durable and timeless.