Actually, RJ45 is just one of the possible pin layout combinations for an 8-pin, 8-conductor (8P8C) modular plug. What's interesting though, the actual Registered Jack (RJ) 45S has really nothing in common with the one people are usually referring to. The original RJ45 had only pairs 1 and 4 connected, whereas 10BASE-T and 100BASE-TX Ethernet use pairs 2 and 3. 1000BASE-T uses all four.
So, basically, in the real world, when people say RJ45, they just normally mean 8P8C connector. It has become so popular these days that even Telco techs starter to call their RJ48 jacks RJ45 because they look the same, and, essentially they are the same just until you actually wire them.
Dmitri Abaimov, RCDD