Keyword = Broad match Your ad will always be displayed if someone search on the keyword you bid on, even if the keyword phrase he search for contains many other keywords as well. "Keyword" = Phrase match Between broad and exact match. [Keyword] = Exact match Your ad will only be shown if the keyword you bid on EXACTLY match the keyword someone is searching for in Adwords. Exact match is the most targeted and the one i use most of the time. Be careful if you select broad match as your ads might be dispayed in the wrong context and you have to pay dearly for it. Only use broad match if you bid on a keyword phrase of at least two keywords. / Peter
The keyword types are a very important factor for new users to grasp. Like Binko said, you can waste a lot of money on Adwords before grasping those concepts and how to use them.