Activation checks are normally issued for a small amount of money, and cashing them obligates the recipient to something. For example, you might receive a $2 check from a phone directory company. When you endorse the back and cash it, you are actually signing up for a listing in the directory. This service will have a fee.
Activation checks have also been used to allow telemarketing calls, switch long-distance service and many other services.
Those who receive activation, or live, checks usually don't realize they are signing an agreement with a company, and are usually not interested in the service they are accepting.
Sample activation check
Front of check
Back of check