"RJ" jack explanations and diagrams

Jacks that connect directly to the phone company have RJ designations. RJ stands for Registered Jack, and refers to FCC-established standards.

A single-line jack for a wall phone is an RJ-11W. A two-line jack for a desk phone is an RJ-14C.
The RJ designation refers to the way a particular piece of hardware is connected at a particular time -- it is not a part number. An RJ-11C, RJ-14C, and RJ-25C can be physically identical, but differ in the number of phone lines connected to them.
Most people call an 8-wire jack used for a phone or a computer network an RJ-45. That's a mistake, because an RJ-45 is a jack used to connect a data terminal to a phone line, but since the same piece of hardware can be used for terminals, networks and phones, any 8-wire jack is commonly called an RJ-45.
The W in RJ designations stands for wall. Nobody seems to know what the C stands for.
There are other suffixes, including X. RJ21X is a common phone company demarcation point (demark) for up to 25 lines

Diagrams below are adapted from material provided by hardware maker Suttle