RJ ... 'Registered Jack', telephone connection interfaces, registered with the FCC, the U.S. Federal Communications Commission. Their history goes back to the Bell System and AT&T's Universal Service Order Codes (USOC) in the 70s. These were adopted later as part of the FCC regulations.

Registered Jack	Width (Positions)	Pins Installed	Installed Pins
RJ-10	4 Position	4 Installed	1,2,3,4			telephone, data
RJ-11	6 Position	4 Installed	2,3,4,5			standard telephone connection, data
RJ-12	6 Position	6 Installed	1,2,3,4,5,6			telephone, data
RJ-14	6 Position	4 Installed	2,3,4,5	connection for 2 analog phone lines
RJ-45	8 Position	8 Installed	1,2,3,4,5,6,7,8			ISDN, LAN, data
RJ-48	10 Position	8, 10 Installed	1,2,3,4,5,6,7,8, 9,10			DATA

The RJ48 has an extra tab so that a RJ48 plug will not push into a RJ45 jack, but a RJ45 plug will mate with a RJ48 jack.