Twisted pair
Thin-gauge copper wire, each insulated by differently-colored plastic for protection from breaking [and water?]. The colors are only to know matching wire at ends of cable.
Hard-drawn copper wire: low electrical resistance, high tensile strength, high resistance to corrosion.
Wire has electrical resistance which reduces the current, thus attenuation.
A pair to reduce external interference eg. lightning (from 19th century telegraphy). Current flows in opposite directions.
Twisting the 2 wires together is to reduce crosstalk from other pairs in cable/bundle (at CO [central office] 19th century telephony). Twist rate is part of specification. Current flow induces magnetic field outside the wire; twisting cancels out the magnetic field of the other wire. A double helix :)
Telephony: voice-grade twisted pair (Cat 1?) in local loop (POTS, ISDN, DSL, T1). residential wiring is untwisted? "silver satin"?
Used in Ethernets from 10Mbps to 10GbE.
Advantages: cheap, physically flexible, good bandwidth, typically pre-installed in new buildings.
Disadvantages: EMI and attenuation limit distance and bandwidth

PVC jacket/sheath. Plastic insulation around each copper wire, here exposed for illustration purposes only (never strip off the colored insulation):

Cables: UTP and STP
UTP (unshielded twisted pair): 4 pairs (8 wires). usually solid core for bulk/horizontal cable, stranded (more flexible) for patch cords/cables/leads.

UTP categories: TIA/EIA-568-B. less attenuation, less crosstalk.
Cat 3 (obs.): 10Base-T. 3 twists/foot
Cat 5: OK for 100BASE-TX. 3 twists/inch on 24 gauge wire.
Cat 5e: better than Cat 5 for 1000BASE-T/full-duplex gigabit Ethernet.
Cat 6 (2002): better for crosstalk and noise. 10Gbps@55m. 22-24 gauge. sometimes internal separators to reduce crosstalk
Cat 6a: 2008. UTP for 10Gbps@100m. Can be flat.
"Cat 7" proposed: pairs shielded and cable shielded. 10GbE
Connectors, wall plates / face plates, patch panel must be same rating. Certify.
Max propagation delay: 548 nsec/100m. Different pairs have different twist rates, thus different lengths, thus different propagation delays.
Do not cut, bend, stretch, crush.

plenum-rated cable/sheath: slow-burning, fire-resistant/flame-retardant casing that emits little smoke for cable in the plenum (space between ceiling and floor, airflow not in ducts). teflon? Not riser cable.

RJ-45 (8P8C) connector/plug into port/jack. Terminates the cable.
All 8 insulated wires visible at very end of it; jacket inside it. <=1" of jacket stripped, <=1/2" untwisted (Cat6:<3/8" untwisted).
gold-plated pins. Cable wire contacting pin for metal-to-metal electron flow.
Factory-made cables with molded connectors. snagless.
Snap tab to lock into jack.

What color wire to what pin of RJ-45. Wire-to-pin assignment (pinout) as per 568A/B spec.:
straight-through cable: same wires at each end. For connecting NIC to hub/switch.
cross-over cable: wires 1 and 2 (oranges), and 3 and 6 (greens) switched at each end (1-3, 2-6). Essentially a 568B at one end, 568A at other end. For connecting two NICs or two hubs/switches.
Auto-MDIX (Auto Sense) (in all gigabit and newer 10/100Mbps) renders distinction obsolete.
10/100Mbps: wires 1 and 2 Tx, 3 and 6 Rx (4578 unused).
1/10Gbps: all 4 pairs used and each pair used to both Tx and Rx at 250 Mbps.

Make your own patch cable with cable jacket cutter & stripper (or scissors); untwist, straighten, flatten and align wires in chosen order (568A or 568B); blade to cut aligned wires; insert into RJ45 module; crimper for squeezing the connector so that cable wires contacting the pins.

Cable tester: continuity: no shorts on each wire; what wire to what wire on other end; Better quality/expense tester: certify: cable at max capability of spec? amounts of crosstalk, attenuation, resistance, interference. Water corrosion at wall plate, broken wires at patch panel.

Ethernet AutoNegotiation. Pulse every 16ms when not transceiving. Also, sometimes?/periodically?/at connection? send 1 or 2 bytes of info about speed and duplex capability.

NIC LEDs (light-emitting diodes) for LiNK/CONNectivity (something live on the other end) and ACTivity (frame input or output). (older sometimes had 10/100 speed indicator)

STP: each pair shielded for EMI protection. for electrically noisy environments eg. factories. Proper grounding of shield at wall plate is important.
aluminum, or better, braided copper, foil.
Screened UTP or STP: outer metal screen for more EMI protection.

Cable installation:
external installation: in room(s) to hub/switch or to wiring closet. Molded floor protectors, raceways and conduits, staples and clips, cable ties or velcro. drilling between rooms and floors problematic.
internal installation (professional): cables are run inside walls, floors, ceilings. Wall/face plates installed. Cables connected to patch panel in wiring closet.

patch cable from computer to wall plate jack. Wall plate has no electronics. Bulk cable ("horizontal cabling") pulled from there to wiring/"telecom closet/room's" 110 patch panel. [66? punchdown block is for telephone cables.]. Patch panel has no electronics. A patch cable from there to hub/switch.
NIC--patch cable--wall plate--bulk cable--patch panel--patch cable--switch port
horizontal cable <= 90m. patch cables total <=10m.
Also, maybe, "backbone cabling" to "equipment room" eg. servers, and to "entrance facility" where network enters the building.
Pulling cable tools: cable puller, telepole, fish tape, ball of string.
Fast&Gigabit Ethernets: only two switches daisy-chainable? but stackable switches many?

Inside of a wall plate: bulk cable directly connected (punched down). Front has RJ-45 jack. E.g. inside wall plate and at other end of bulk/horizontal cable in telecom closet's patch panel.
Wiring closet: On left, switches in equipment rack. On top right, bundle of bulk cables from rooms' wall plates of this floor of building. On right, each cable is punched down on back side of patch panel, on the front side of which are RJ-45 jacks.
Each wall plate jack is labelled with some identifier, matched with same on patch panel jack. If unknown, use toner to locate.

Lack of connectivity between PC and hub/switch:
  HW: bad cable (3) or termination (6), bad NIC/port (corroded, unseated, fried), device powered off, plug not fully in jack.
  SW: bad/unistalled driver, port disabled, speed or duplex mismatch,

Gigabit 6 port NIC
Structured cabling
Building backbone