Twist pair cable
RJ-45 or 8P8C modular connectors
The most popular connectors found these days on the ends of network cables are commonly called RJ45 connectors.
An RJ45 connector has 8 conductors. it has the naming convention of 8P8C (8positions 8conductors).
"RJ45" generally refers to 4-pair UTP (generally CAT5e or CAT6) terminated with 8P8C connectors to the TIA/EIA-568 standard. Ethernet is by far the most common layer 2 technology to utilize this type of cable.
A network card, or network adapter as it is sometimes called, allows a computer to connect to a network of other computers. These devices will usually have both a physical layer and a data link layer. Ethernet devices will most often have an 8P8C socket where the network cable can be connected.
The 45 indicates the jack number; RJ45 indicates Registered Jack number 45. However, there is a slight complication - what most people call RJ45 should actually be 8P8C. The RJ numbers used to be allocated by the FCC, but they are now defined by Administrative Council for Terminal Attachment (ACTA).
RJ-45 is the standard connector utilized on 4-pair (8-wire) UTP (Unshielded Twisted Pair) cable. The RJ-45 connector is the standard connector for Ethernet, ISDN, T1, and modern digital telephone systems. RJ stands for "Registered Jack". Be aware that the Ethernet UTP connector is properly called an "8P8C".
An RJ45 connector and a PCI NIC card are 2 completely different things. An RJ45 (Technically an 8P8C) is the connector at the end of a CAT cable which plugs into a NIC card. Unless you are thinking RJ11, which has nothing to do with CAT cables. An RJ11 is the name of the cable used for single-line dialup modems and telephones, in which case, the connector is called a 6P4C connector.
If it's the common "CAT5", "CAT5e" or "CAT6", it's around 1/4 inch thick, blue, grey or purple jacket, and slightly bumpy due to the four pairs of twisted wires inside the jacket. The connectors are most likely the rectangular clear plastic plugs known (incorrectly) as RJ45, and correctly as 8P8C, fitted to the cable with a special crimping tool, die tool. The cable can be solid-core (cheaper, only suitable for wired-in cabling) or stranded (more expensive, suitable for patch cables and panel-to-computer cables). When fitting connectors, be sure to use the correct ones (for solid core or stranded core).
You use a Rj-45 connector with twisted-pair cabling in an Ethernet LAN. Rj-45 is a connector with a 4 5 wiring sequence.
A rj-45 network jack is to connect your printer to a wireless hub/router with a cable therefore making it wireless. If your printer already has a network adapter installed, it is considered network-ready. If your printer is ready for a wired network, you'll see an RJ-45 jack (which looks like a wide telephone jack), on the back.