The single user license is designed to be used by one person on their own personal machine. This license is tied to the user.
A floating license is designed so that it can be moved to different machines and users within a company. The license is tied to the company and can only be used by company members. A single floating license can only be installed on one machine at a time. Registering the license on a machine will automatically unregister it from the previous machine. Multiple floating licenses can be purchased at a discounted rate.
A site license follows the same rules as a floating license, except that additional licenses can be added as required.