Approximately 450,000 usernames/emails and passwords were compromised by a hacker using a well known exploit on what appears to be a Yahoo Voice Service web site.
You can easily search to see if you were compromised by going to this website: http://labs.sucuri.net/?yahooleak and entering your Yahoo email address. If you are part of the leak, you need to immidiately change your password as well as any other accounts which use that password.
Additionally, if you have linked any other emails to that account, you should change those passwords also. This example is why you do not want to share passwords between accounts.