Zee, a Google AdWords representative, posted in the Google AdWords Help forum an interactive infographic on the five stages of making a decision on your vacation/travel plans.
The point is for advertisers to be able to reach these travelers at the stages that are most applicable to their service offering.
Zee explained "with the Internet as the leading source for travel planning, the AdWords advertising is one of the tools used by travel businesses to reach their potential clients during their buying cycle."
Here is the info graphic:
Forum discussion at Google AdWords Help.