The only reason why you need to use this tool is to check progress. This tool will tell you how many pages of your website are indexed in Yahoo.

To know how many pages you have in Google I recommend you to signup for a FREE account at Google Webmaster Tools...

That account will provide you lots of other tools and with just one account you basically have everything. You'll also know how many pages are indexed in Google.

Please refer to the SEO Videos on how to create and submit a Google sitemap.xml file - only after you create and submit a sitemap.xml file will Google provide you lots of detail about your website.

You will also notice that probably in Yahoo you have more pages indexed than in Google. That's normal, not all search engines are the same.

To know if you're progressing well just make sure that almost all of your website pages are indexed. If you have 500, then around 420 should be indexed. The FREE Google webmaster tools account will tell you how many pages you have on your site and how many indexed. 

So I highly recommend you follow the SEO Videos tutorials about creating and submitting a Google sitemap.xml file so you'll get to know the numbers and data about your website - to know what's going on!

If you have 500 pages but only 200 are indexed then probably you have issues that need to be resolved. In most cases duplicate content issues. If you follow proper SEO techniques such as the ones inside the SEO Booklets - you'll not encounter such issues!