UART is asynchronous -- unlike SPI or I2C there's no shared clock line at all. Both sides just have to agree on a baud rate up front, and each byte is framed with a start bit and one or more stop bits so the receiver can tell where it begins and ends. It's point-to-point: TX on one device wires to RX on the other and vice versa, not a shared bus multiple devices can sit on.

The DB9/DB25 pinout below is RS-232 -- the traditional PC-side connector and voltage-level standard for carrying UART signals over a cable. Microcontrollers almost never use RS-232's voltage levels directly; they expose bare TTL-level TX/RX pins instead, and something like an FTDI converter bridges that to USB.

The pin layout is viewed from the front and the solder points pointing away from you:

Most of the time only GND, TxD, and RxD is needed. The next level of handshake is RTS and CTS. Very rarely DTR and DSR is necessary. I never used DCD or RI.

Signal		Origin				Pins
Abbreviation	Name	Importance	DCE (in)	DTE (out)	DB 9	DB 25
GND	Ground	***	-	-	5	7
RxD	Received Data	***	x		2	3
TxD	Transmitted Data	***		x	3	2
CTS	Clear To Send	**	x		8	5
RTS	Request To Send	**		x	7	4
DSR	Data Set Ready	*	x		6	6
DTR	Data Terminal Ready	*		x	4	20
DCD	Carrier Detect	-	x		1	8
RI	Ring Indicator	-	x		9	22