pam_usb is a PAM module that allows you to log in to your Linux box using a mobile USB storage device such as an USB pen.
Depending on your pam configuration, it can be set up to work in three different modes:
Unique security: You can log in only using your USB pen.
Alternative security: Plugging in the USB device is enough to log in. If its not present, the system prompts for password.
Additional security: To log in you have to plug in the USB device and type your password.