The two common types of phone jacks are called RJ-11 and RJ-45 (RJ means registered jack, a designation of the FCC). RJ-11 is a basic jack and is used for a single phone line and for dial-up internet service. RJ-45 is slightly different in that it has larger bandwidth and is used for ethernet cables. Other types of phone jacks and plugs are available in different sizes and used as connectors for audio systems (such as amplifiers and speakers).
