While having a quick look at some Google rankings I came across this advert from Wickes, the DIY store, in the paid listings:

It looks like Wickes have bid on "DIY" using either broad or phrase match.  This is something to be wary of when using Adwords.  It may not be too great an issue for Wickes as most people searching for phrases containing "DIY" are searching for a product Wickes offers, "DIY Floors" and DIY Tools" are some examples.

Consider the same situation when applied to cars though, imagine you're the owner of a showroom selling high end cars, Ferraris, Porsches and the like, and you bid on "cars" using broad or phrase match.  Chances are you'll start appearing for queries such as "cheap cars", "used cars", "budget cars" etc.  People searching for these clearly aren't your target market and are unlikely to click on your ads which, in turn, is going to impact your quality score and the chances of your ads appearing above your competitors'.  For these situations use an exact match to only appear if the exact keyword you're bidding on is typed into a search engine, alternatively use negative match to ensure your ad DON'T appear for queries containing selected keywords.

So when it comes to Adwords make sure you know your market AND the Adwords interface for the best chance of getting click-throughs and good value, now, I'm off to buy some hammers.

There is lots of great info about Adwords at Google Adwords help, we also talk about Adwords in our one day SEO Training course.