Shareware is a way to sell software.
The user will get a preview or demo of the software for free. This way, the user can decide if he or she likes the program or not before buying the complete program.
Users can get games for free this way but most of the time these games do not have all the levels. Sometimes Internet Relay Chat clients let users have a demo of their client for 30 days, but after that, in order to keep using it, the user will have to pay money.