Yahoo! has just announced a new feature for webmasters that lets them define dynamic parameters within URLs. Why is that a handy feature to have? From the Yahoo! Search Blog:

• A more efficient crawl of your site, with fewer duplicate URLs being crawled.
• Better and deeper site coverage, as we’ll be able to use our crawler capacity to find and index more new content on your site.
• More unique content discovered, as we’ll handle more dynamic parameters in your URLs (if you remove the content-neutral dynamic parameters).
• Fewer chances of crawler traps.
• Cleaner and easier-to-read URLs displayed in the search results.
• Better aggregation of link juice to your sites, which can help your pages rank better.

Yahoo’s help pages explain it a little more clearly:

This feature allows you to specify what parameters on your site are dynamic parameters that are content-neutral (that is, they do not effect the content on the page) and are present for other uses. To help you get started, we also show you the parameters that we found as possible candidates in our analysis of your site crawl. You can select one of these parameters and provide actions for them.

Let’s say, for example, that every time someone visits your site you assign them a session ID. That ID is then attached to every URL that the visitor sees. For example, www.yoursite.com/articles/somearticle.html&sid=001 is article #123 being viewed by someone who’s been assigned session ID 001. If you tell Yahoo! that “sid” is a session ID, and that you’d like it dropped, their crawler will rewrite the URL in the SERPs to read www.yoursite.com/articles/somearticle.html.

Yahoo! Help has more detailed information, examples, and specifics of how the process works.