A logo is the calling card for a company or service.
Some of the best logos are ones that are recognizable without seeing the name of the company.
You get the idea.
Some have developed to change with the times, occasionally so subtle you don't even realize it's changed until you see it's previous versions:
These examples make good use of negative space. From time to time, what you don't say is just as important as what you do.
Bad logos, while potentially damaging to a company's brand, are often entertaining.