Bits have to be moved from one place to another using wires or some other medium. Over many miles, the expense of the wires becomes large. To reduce the expense, several bits, from five up to several thousand, are sent together on a single wire or optic fiber.
By convention, teletype-style UARTs send a "start" bit, five to eight data bits, least-significant-bit first, an optional "parity" bit, and then a "stop" bit. The start bit is the opposite polarity of the data-line's normal state. The stop-bit is the data-line's normal state, and provides a space before the next character can start. In mechanical teletypes, the "stop" bit was often stretched to two bit times to give the mechanism more time to finish printing a character. The parity bit can either make the number of bits odd, or even, or it can be omitted. Odd parity is more reliable because it assures that there will always be a data transition, and this permits many UARTs to resynchronize.
Standard speeds for teletype UARTs are in changes of the data-line per second, or baud. Standard mechanical teletype rates are 110 and 150 baud. Computers have used from 300 to 56,200 baud. A standard speed for computers is 9,600 baud, which fills a screen with characters in less than a second.
Teletype UARTs use several different wiring and voltage standards. "Full duplex" standards permit both stations to send and receive at the same time. "Half duplex" arrangements permit only one station to talk at a time.
The most common, RS-232, uses two wires to send full-duplex. Each wire has plus twelve volts for mark, and minus twelve volts for space. RS-232 uses a rather large number of auxiliary lines (data carrier detect, data terminal ready, data set ready, request-to-send, clear-to-send), to signal that a connection is offered or refused. Often these are omitted, and a so-called "RS-232" connection will just have transmit, receive and ground.
Another common standard is RS-422. This is "differential", that is there are two wires, both data, and the difference in their voltage conveys the state of the data. The difference is supposed to be five or minus-five volts. The differential scheme means that electrical noise, when induced equally on both wires, will not corrupt the data. RS-422 is a point-to-point unidirectional transmission system. A full-duplex standard using the same differential scheme is called RS-485.
The word "asynchronous" distinguishes UARTs from digital telephonic systems that use synchronous frames of bits that repeat 8000 times per second. These usually have a one or a few bits that form a pattern to enable the system to "synch", and locate the start of the frame. This is far more efficient than a teletype-style UART, because there are fewer bits used for overhead. Telephone systems range from T-1 lines, which have a frame of 24 bits, to SONET frames, whose frame contain several thousand bits.
Some UARTs are able to send synchronous streams of characters, to be multiplexed into a telephone-style data transmission. These are called USARTs (for "universal synchronous asynchronous receiver-transmitters"). The usual filler is the ASCII "SYN" character.
Later, ASCII required a seven bit code.