Can the webmaster know a user's password?

No.

Passwords are stored in an encrypted format, not the string you entered. When you enter your password, the system just encrypts it and compares the encrypted string with the one stored on the system (in MySQL). If the strings match, you are allowed to login.

However the webmaster can change the password for you should the need arise.

The encrypted string is something like this:
e309ca8ebf7fdaceedc3e92bc430d780
(Some characters have been changed.)

This Q&A was found on Language Fun : http://okenglish.tw/modules/smartfaq/faq.php?faqid=8