What is the difference between T568A and T568B?

They are two RJ45 wiring color schemes. Pin functions are identical; only the green and orange pairs swap positions. Pick one standard and use it on both ends for a straight-through cable. A crossover cable uses T568A on one end and T568B on the other.

Which way do I count pins on a connector?

Pin numbering is defined from a specific viewing side, usually looking at the mating face of the male (plug) connector or the solder side of a header. The female (socket) is mirrored. Always confirm the datasheet orientation before wiring, because a mirror flip swaps every pin.

What are the four pins on a USB Type-A connector?

USB 2.0 Type-A has four contacts: VBUS (+5V, pin 1), D- (pin 2), D+ (pin 3), and GND (pin 4). USB 3.0 adds five more pins deeper in the connector for SuperSpeed pairs.

What is the difference between CTIA and OMTP 3.5mm wiring?

Both are 4-contact TRRS schemes. CTIA (the modern standard used by most phones) wires the sleeve to the microphone and the second ring to ground. OMTP swaps them. A mismatch makes the mic not work and audio sound muffled.

Are the Raspberry Pi GPIO pins 3.3V or 5V?

The GPIO logic is 3.3V and is NOT 5V tolerant. Pins 2 and 4 supply 5V power and pins 1 and 17 supply 3.3V, but driving any GPIO input above 3.3V can damage the chip.

Reference

Connector Pinouts

Pin-by-pin reference for the connectors you wire most: USB-A, USB-C, RJ45, DB9 RS-232, HDMI, 3.5mm audio, and the Raspberry Pi GPIO header.