If keywords are the blood of your campaign then your adverts are the beating heart.
Writing a good advert is an art. Writing a good advert using AdWords gives you a science on which to base the art.
You have a total of 95 characters to convey your message: 25 in the headline and 35 in lines 2 and 3. There are also 35 characters for a display URL but you can’t use this for ad copy. For something so small it’s surprising what you can do with it as well as how much subtlety you can apply to it.