The RJ-45 connector is commonly used for network cabling and for telephony applications. It's also used for serial connections in special cases. Here's a look at it:
Pinout for Ethernet
Although used for a variety of purposes, the RJ-45 connector is probably most commonly used for 10Base-T and 100Base-TX Ethernet connections.
Because only two pairs of wires in the eight-pin RJ-45 connector are used to carry Ethernet signals, and both 10BASE-T and 100BASE-TX use the same pins, a crossover cable made for one will also work with the other.
Also, please note that it is very important that a single pair be used for pins 3 and 6. If one conductor from one pair is used for pin 3 and a conductor from another pair is used for pin 6, performance will degrade. See the following figure.
To learn more about Ethernet, check out Charles Spurgeon's Ethernet Reference.
RJ-45 Pinout for RocketPort
The following chart shows the pinout for RJ-45 connectors used on certain RocketPort serial interface cards (manufactured by Comtrol).
Pinouts for ISDN
Here's an ISDN BRI U port pinout for a Cisco 750 series router:
The following chart shows the pinout for RJ-45 connectors used on certain ISDN S/T interfaces. For more info, see ANSI T1.605.