Where do we get all of this knowledge about keyword demand and keyword referrals? From research sources like these listed here:
Google Adwords’ Keyword Estimator
Google Insights for Search
Google Trends Keyword Demand Prediction
Microsoft AdCenter Keyword Forecasting
Wordtracker’s Free Basic Keyword Demand
We can see that Google is predicting both the cost of running campaigns for these terms as well as estimates of the number of clicks a campaign might receive. You can use these latter numbers (under the "estimated clicks/day" column) to get a rough idea of how popular a particular keyword or phrase is in comparison to another. The green, horizontal bar in the "search volume" column can also help to show comparative estimates of demand.
Other, less popular sources for keyword information exist, as do tools with more advanced data, and these are covered excellently in the Professional's Guide to Keyword Research, which will teach you how to do keyword research.
