To login, simply click the login link at the top left hand corner of the website.

To log out, click your profile picture in the top right hand corner of the website and click "Log Out". In the event you cannot log out, clearing cookies on your machine will take the same effect.

When you login, you set a cookie on your machine so that you can browse the forums without having to enter in your username and password each time. Logging out clears that cookie to ensure nobody else can browse the forum as you.