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Let's start by defining 'keywords' themselves.

The easiest approach is to see them as search terms - the words or phrases that are typed into a search engine such as Google or Yahoo!. Any keywords used in searches can then be measured to find out how often they're used.

It would be a fairly safe bet to assume that more people look up "latest movie releases" than "movie subtitle services", but keyword analysis tools take away the guesswork by telling you exactly how popular a keyword or phrase really is.

Such tools are widely available free online (we've links to some of the best in our Tools section), and all work in roughly the same way: you simply type in a keyword query, press a button and receive a percentage-based reading showing the amount of online usage.

From an affiliate marketer's point of view, the important thing about popular keywords is that advertisers pay more to use them. Which also means that, if your pages feature ads containing these words or phrases, you should be paid more for clicks or views too. (In fact, Google's own keyword analysis tool gives approximate keyword click prices so you can see how much this amount is likely to be).

So let's imagine a possible scenario in which you'd use this information to optimize your earnings.

We'll say that you're running a travel site and want to set up a feature article about city breaks. Before you even begin writing, however, you use a keyword analysis tool to check out the most popular - and therefore highest paying - keywords.

By typing in, say, 'New York' and 'Las Vegas', you discover that Vegas is currently a much more popular search term, with ads costing substantially more than the New York option.

What's more, by experimenting a little with different combinations of wording, you also find that the most expensive keywords of all are currently 'Las Vegas hotels'. So you now know that an article focusing on this topic should attract plenty of higher paid contextual advertising, with more cash per click than if you'd written about New York.

What's more, since the subject is popular in search engines, you'll be providing readers with an article that's geared to current interests. It's a win-win situation.

Some affiliate marketers will go even further and do keyword research before they publish a website. The results will then be used to create a site specifically tailored to particular high-paying keywords or phrases.

Remember, though, that the popularity of keywords can fluctuate quickly, (which is another reason why keyword analysis tools are considered so important).

But whatever approach you decide to take, we hope we've helped you unlock those keyword mysteries - and opened the door to higher online earnings.

More info - how to refine keyword use in Google