Most pay per click performance measurement has been at the keyword level i.e. which keyword resulted in conversion.
The problem is this is a blunt measurement tool. It would also be great to know what steps led up to that keyword. For example, did the visitor search on a number of different keyword variations before s/he arrived at that particular keyword. Up until now, this type of tracking has been painful to implement.
However, Google has just released Search Funnels in Adwords, which should make things a whole lot easier.
Search Funnels are a set of new reports describing the Google.com search ad click and impression behavior leading up to a conversion. Currently, conversions in AdWords are attributed to the last ad clicked before the conversion. Search Funnels gives advertisers data on how “upper-funnel” keywords are assisting conversions before the last click. It also enhances basic conversion reporting for AdWords.
So, advertisers can see which searches led up to the purchase, and can bid on these terms, too. Also helps Google’s bottom line, of course, by driving up the cost of keywords that may not be obvious 
For example, someone might search on “Ford lights” and visit your site. They don’t buy anything. A while later, they return to your site after having searched on “Fiesta parts”. They finally convert, purchasing a Ford Fiesta tail light. The funnel would show that the keyword term “Ford lights” assisted in the conversion, even though it didn’t directly lead to the sale.
How is Google tracking this data?
When someone clicks on an ad at Google, Google starts a funnel, although you can’t see it at this point. Search activity is tracked for 30 days. If the user conduct other searches, even if they don’t click on the advertiser’s ad, Google will add those searches to the funnel, so long as the advertisers ad was displayed.
If the user clicks on the advertiser’s ad and converts, then a funnel report is created, and this data is shown in your Adwords account. You need to use AdWords conversion tracking code for this to work. Google is logging the time and date of every search by that user prior to a conversion, and once a conversion occurs, all that data is available in your Adwords account. If no conversion occurs, obviously you won’t see any search funnel data.
Having the ability to track keyword paths across time is going to open up some fascinating data. As Google points out:
Search Funnels also show the “Path Length,” or average number of clicks and impressions prior to conversion, which can help you understand and target repeat visitors. Additionally they show “Time Lag,” or the amount of time it takes a customer to convert after seeing or clicking on your ads for the first time, which can help you determine lead time for seasonal campaigns. These breakdowns can help you understand your customers’ behavior and create strategies around that information.
Now we can track (roughly) how long non-impulsive purchasers take to arrive at a buying decision.
While this shines more light on the keyword research process, there will still be chunks missing. Any search that doesn’t result in an advertisers ad being displayed won’t be tracked. If the user searches elsewhere, using another engine or uses bookmarks or other navigation methods, obviously this won’t show up in the conversion funnel either.
There’s also the problem that assist clicks might not count for much. It might be difficult to establish repeatable patterns when multiple keyword searches are involved. High volume advertisers will probably receive the most benefit, as there is more chance of repeatable patterns showing up.
Here’s a Google walk-through of the reports:
