Think about McDonalds and you will see the yellow arches, in the shape of an "M". Think of Nike and the swoosh is the first image in your mind.
It doesn't have to take millions of dollars and 15 marketing consultancies to come up with a great logo, but make sure you get a professional designer to make it.
The characteristics you should be after:
If you follow these principles, people should be able to look at your logo from a distance and know which company it's from. The best trademarks end up cemented into the minds of individuals worldwide.
Some examples of great logos:



It is very important to remember that your logo should be consistently displayed wherever your company promotes itself.