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.