Plain passwords

Why in my DB (MySQL) many users still have theirs passwords in plain text, not encrypted?

I’'m using the Wildfire 3.2.2 on CentOS 4.4 (rpm)

How is the conversion from plain to encrypted?

I want to be able to access the DB direct, and compare the password given by the user (plain text) with the real password (encrypted). Can I do that from the Linux console (shell script)? If not, how can I do?

Thanks,

Rodrigo.