Operating systems and other programs sometimes use activation codes to ensure that an installation is properly licensed. Some programs only require that users enter the code once, while others use the code to check a server every time the program runs.
Barring special measure, software is easy to copy, and the Internet makes finding illegal software even easier. One method companies use to combat software piracy is requiring activation before users can run the program. Activation codes allow the program to check if the user has a valid key. In some cases, the program doesn't activate unless it can access a server to confirm that the supplied activation code is valid.
Windows traditionally uses activation codes, but users can generally run Windows for a certain period of time before they have to activate it. Activation codes are more popular on expensive programs from companies that have complex licensing programs. Clients who order programs for multiple workstations or servers might only have to enter a single activation code.
Activation codes are sometimes used for two-factor authorization or for extra security. When a user accesses a resource on a new computer or mobile device, the service provider sends an activation code to his phone or email address. This prevents others from accessing the account even if they gain access to the user's password.