The connector was originally used for telephone equipment. The RJ means Registered Jack.
The six conductor connector (sometimes only the center two or four are used) that is typically used for connecting telephone equipment. Widely used in DCC for bus connections and cab plugins.
The RJ11, 12, 14 and 25 are the same physical size, the difference is the RJ12 is a 6 position, 6 conductor device. The RJ25 is the same, except it is meant for stranded wire. See the LocoNet article for more information on the RJ connector series.