What are the differences between RJ connectors?

RJ45: 8P8C = 8 Position 8 Contact - used in Network Wiring
RJ12: 6P6C = 6 Position 6 Conductor - used in System phones
RJ11: 6P4C = 6 Position 4 Conductor - used for ADSL, telephone, and modem cables etc.
RJ10 / RJ22: 4P4C = 4 Position 4 Conductor - used for Telephone Handset cables.

Above is a list of some of the common connector types for terminating communication wires like your cat cable that you use for internet.  Each one is used for a different purpose and vary is size.