Google AdWords offers three different matching options to determine if a search query will trigger your ad to be shown.

The first is known as exact match. In your Google AdWords keyword manager, these words are bracketed. [keyword example].

Exact matches are shown only when the searcher inputs this exact keyword. If you bid on [Mexico travel], the search for Mexico travel will trigger your ad. However, Mexican travel would not.

Exact keyword matches can be useful if

? You are a very niche site, trying to strictly control your budget.
? Wish to attempt using broad keywords (i.e. the keyword travel), but do not want to be shown for every search containing the keyword travel.
? Have thousands of keywords and trying to figure out which ones are giving you positive and negative ROI.

Exact keyword matches can hinder a sites ability to find new traffic sources or new keyword combinations as you are severely limiting your ads visibility.