Serial v.s. Parallel [1][2]

Serial communication data is transferred serially (one after another) and not parallel (everything together). So as expected, a serial communication can be done using fewer wires as compared to its parallel counterpart and it also needs some sort of syncing mechanism (clock) to make a successful communication.

Asynchronous v.s. Synchronous [1][2]

Synchronous serial communication: In this type of communication both transmitter and receiver share a common clock to remain in sync with each other. Asynchronous serial communication: This type of serial communication does not require any common clock source between the transmitter and receiver, both the sides work according to their independent clocks.

Serial Communication on Arduino Uno

Serial Communication on Arduino Nano

Serial Communication on Raspberry Pi (RPi) [3]

Serial Communication: UART v.s. I2C v.s. SPI

	UART	I2C	SPI
Full Name	Universal Asynchronous Receiver Transmitter	Inter-Integrated Circuit	Serial Peripheral Interface
Connection
SYN	Asynchronous	Synchronous	Synchronous
Wires	2 (RX, TX)	2 (SCL, SDA)	3 (SCLK, MISO, MOSI) + N (SS)
Devices	1 to 1	N Masters, N Slaves	1 Master, N Slaves
Receiver and Transmitte	Simplex or Half-Duplex or Full-Duplex	Half-Duplex	Full-Duplex
Speed	9.6/19.2/38.4/57.6/115.2 kbps	100/400 kbps	a few mbps
Distance	around 15m	around 1m	around 0.2m
Power	medium	medium	lower
Error Detection	parity bit	no	no
Flow Control (Acknowledgement Mechanism)	no	ACK bit	no
Data Start From	LSB First	MSB First	LSB or MSB First
Data Transmission	Frame (start bit, data bits, parity bit, stop bit)	Address (start bit, device address 8 bits, ack, internal register address 8 bits, ack, data 8 bits, ack, stop bit)	Slave Select/Chip Select. Active low, it is pulled high to disconnect the slave from the SPI bus.