Phreaking is a slang term that describes the action of experimenting with or manipulating a telephone system. Since phreaking took place before personal computers became popular, it is sometimes considered to be the precursor to computer hacking.
While not all phreaking activities are illegal, the term is often associated with using a phone system to make free long distance calls. Early phone systems has limited security features, which allowed "phreaks" to tap into analog phone lines and make calls free of charge. This was often done using a device called a "blue box," which simulated a telephone operator's console. Phreaks could use these devices to route their own calls and bypass the telephone company switches, allowing them to make free calls. This activity was more prominent before the turn of the century, when cell phone companies began including free long distance service.
Phreaking has evolved over past several decades along with telecommunications technology. In the 1980s and 1990s, phreaks began using modems to access to computer systems over telephone lines. Once connected via modem, tech-savvy users could access private data or exploit computers connected on the local network. This activity also faded out around the turn of the century as dial-up modems were replaced by DSL and cable modems and new security measures were put into place. While phreaking still exists, it is much less common than other types of computer hacking.