A computer program that causes your modem to dial a number to a computer server. This is meant to allow the caller to access/download files from the server.
Almost all dialers are for pornography. They may claim to be free, but always go to long-distance telephone numbers. These numbers are usually equivalent to 1-900 numbers in the U.S. (the numbers vary from county to country). The caller can end up paying a ridiculous amount of money in phone bills.
After using a dialer to download movies of Traci Lords, Jim Bob was shocked to find out his phone bill was $8,000.