We encrypt your password and store it along with your username in a cookie. When you visit the site again later, we take that cookie and use it to automatically log you back in. The password is encrypted with a random TripleDES key, making it highly unlikely that your password could be discovered - even if someone had millions of dollars and a year of their time to devote to the task.
How do I increase security:
When you check the box that says 'Increase Security', we clear the information out of the cookie and set it to expire immediately. When you close your browser window or are inactive for 20 minutes your session will expire and you will be required to login again. This is useful and recommended for browsing on public terminals or in sensitive areas where others may have access to the computer. Although your password is still encrypted, anyone could impersonate you on the forums.