An RJ12 is a registered telephone jack with six pins, also known as a 6-position modular connector, which is used for voice communications, specifically analog or telephone wiring. The previous version was RJ11 that used only four pins.
The most-common connector in use is an RJ45, an 8-position modular connector that is more versatile than one with six positions. It is used for telephone wiring in ISDN and T1 applications as well as data applications such as 10baseT and 100baseT. As networking, wireless and cloud technologies continue to grow, there will be a continual decline of RJ11 and RJ12 connectors in use, while RJ45 and fiber technologies gain in popularity.