For the ones who don't know, Site explorer allows you to inspect certain aspects of a URL. The "Inlinks" feature shows you details about web pages that link back to the particular URL you are exploring. Site Explorer gives you some insights on how popular a website is based on how many web pages are linking to it.
It is almost similar to Google Webmaster Tool but with fewer options.
Why we want to use Yahoo! Siteexplorer? Because we, the Webmasters need to aspire to get our project's to index on all the major SE's (More traffic and more popular)
Link for Yahoo! Siteexplorer:
https://siteexplorer.search.yahoo.com
Verify your website and start using it, takes around 12 hours to fully verify your website.
Feel free to post your question regard siteexplorer, Tips and Errors.