RJ stands for registered jack. RJ-XX is a general term for electrical connector designs registered with the US Federal Communications Commission, including the RJ11, RJ14 and RJ45 connectors.
The most familiar registered jacks are the 4-conductor and the 6-conductor connectors known variously as RJ-11, RJ-12 and RJ-14, and the 8-conductor RJ-45, all sometimes simply called RJ connectors. These are commonly used in building wiring for telephone and local area networks.
There is much confusion over the naming of the smaller of these connectors. The six-pin plug and jack commonly used for telephone line connections may be called an RJ-11, an RJ-12 or an RJ-14, all of which properly define interfaces that use this pysical connector. But the smaller four-pin plug used for handset connections is commonly called an RJ-11, to distinguish it from the larger six-pin plugs which are then called RJ-12 or RJ-14. The FCC themselves most commonly refer to the 6-pin connectors as RJ-11. In all of these plugs, most commonly only the centre two pins are connected. These six-pin plugs are often called modular connectors to distinguish them from older telephone connectors.
There is also confusion over the physical compatibility. As designed, they are physically compatible, in that a four-pin plug will fit a six- or eight-pin socket, connecting to the centre four of the conductors, and a six-pin plug will fit an eight-pin socket, connecting to the centre six conductors. However plugs from different manufacturers may not have this compatibility, and some manufacturers of eight-pin jacks now explicitly warn that they are not designed to accept smaller plugs without damage.
All these connectors are normally used with twisted pair wiring. Wiring conventions were designed to take full advantage of this physical compatibility, but here again there has been a problem. The original concept was that the centre two pins would be one pair, the next two out the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Additionally, signal shielding was optimised by alternating the "live" and "earthy" pins of each pair. This standard for the eight-pin connector results in a pinout known as TIA-568-A, but the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols. A variation known as TIA-568B overcomes this by using adjacent pairs of the outer four pins for the third and fourth pairs. The inner four pins are wired identically to TIA-568A.
Left to right: RJ-45 plug, six-pin plug, four-pin plug the same size, four-pin handset plug, six-pin jack