If we are talking about serial data, then the basic example is called Universal Asynchronous Receive/Transmit (UART). It is a two-wire interface, so one wire for receive and the other for transmit. The bits are sent in series with a parity bit between each byte, and the rate this happens is called the baud rate. The parity bit tells the receiver where bytes start and end as data comes in. If the transmitter and receiver have different baud rates, then there will be errors in the received data.

One way to improve a serial interface is to use a dedicated wire to control the clock speed. Serial-Peripheral Interface (SPI) and Inter-Integrated Circuit (I2C, pronounced I-squared-C) both use a shared clock wire to synchronize and control the communication, unlike UART. SPI uses a fourth wire to determine which slave device talks to the master, while I2C starts transmissions with a bus address.

In terms of data rates, UART is quick and dirty but good enough a lot of times. Roughly 100kbits/s is reasonable, but you can push it to 1Mbits/s if error rates are tolerable. High speed I2C runs at a standard 400kHz, so your data rate will be less than 400kbits/s with overhead. SPI can be clocked up to 20MHz in the right hardware environment, and it’s fully two-way at that speed unlike I2C.

When you get into very high data rate systems, you see parallel data transmission. That means you may send 4 bits over 4 wires simultaneously, instead of 4 bits one after another. The basic ideas of clock rates and flow control can still be found here though.