Nagware (also known as begware or annoyware) is a type of shareware that reminds (or nags) the user to register it by paying a fee. It usually does this by popping up a message when the user starts the program, or intermittently while the user is using the application. These messages can appear as windows obscuring part of the screen or message boxes that can quickly be closed. Some nagware keeps the message up for a certain time period, forcing the user to wait to continue to use the program.
The intent is generally that the user will become so annoyed with the messages that he or she will register it to get rid of them. Naturally, there is a fine line to be walked — if reminders are too annoying, users may abandon a program without even evaluating it fully, which is as much a lost sale as a failure to register.
Developers who employ the practice report that shareware with less intrusive reminders garner only a fraction of the registrations of nagware and theorize that if users aren't regularly reminded that they should pay for a particular piece of software, they tend to quickly forget.
Examples of nagware are The Bat!, WinRAR, WinZip, mIRC, and SmartFTP. These programs pop-up a window after their trial period is up telling the user to buy the program.

Reference

Ian Parberry (March 5, 1997). "The Internet and the Aspiring Games Programmer". University of North Texas.