If you've received an email claiming to be from Yahoo! that asks for your password or other private information, you've likely received a "phishing" email. These emails typically contain links that appear to go to a Yahoo! site but actually link to a fake, or "spoofed" Yahoo site.
If you are uncertain about any "official" email you've received, do not click on any links in the email. The safest way to visit a website is to type the site's address directly in the address bar of your web browser.
No Yahoo! employee will ever ask you for your password or personal information in an unsolicited phone call or email message. Learn how to report a phishing email to Yahoo! here. To learn more about phishing, please visit the Yahoo! Security Center.