RJ-11 - The most common type of telephone jack in the world, a 6-pin male modular jack (or plug) that connects to a female jack on a wall (or an RJ-11 adapter). RJ-11 jacks are usually only wired for four pins and only two of them (usually the red and the green wires) are used for the phone signal. The second pair can be used to carry a second phone line or to run low-wattage electronics, such as lights on phones. The second twisted pair of wires are increasingly being used for phone-based home networks (to connect your home PCs through your phone wiring). "RJ" stands for Registered Jack. The numbers that follow RJ (RJ-45, RJ-61X, etc.) are designations assigned by the FCC
RJ-45 - Refers to a type of jack, similar to an RJ-11 (phone) jack, only wider, with 8 conductors, rather than 6. RJ-45 connectors are used for a variety of purposes including networking and telephony.