As Michael said, the UART itself does require a clock; it's just that the clock is not transmitted along with the data.


So, continuing in the pedantic vein, SPI, I2C and Microwire are serial protocols that include a clock along with their data line(s).


In RS232, note that TXD is an output from a DTE, but an input to the DCE; similarly, RXD is an output from a DCE, but an input to a DTE.
See: http://www.8052.com/forum/read.phtml?id=60533


The 'U' in "UART" stands for "Universal." Part of its Universality is the ability to use many different frame sizes - 5, 6, 7, or 8 bits. (The 8051 UART can only do 8 or 9 bits).
As I mentioned before, a UART may also do other stuff like automatic parity generation & checking, automatic software and/or hardware flow-control, etc, etc,...


Actually, the UART will take more samples during the start bit time to ensure that it's a real start bit, and not just a glitch.
Similarly, each data bit is sampled multiple times.
Commonly, UARTs sample at 16x the baud rate.


Again, the number of bits per character may be programmable.
The UART will also sample the stop bit for the specified time to check for a valid stop bit before delivering a valid character.


Again, the character width is programmable.


This is nothing to do with Hayes.
This is a feature implemented by many (most?) modems, having spotted that (almost) every Hayes command begins with an 'A'


It is not necessarily true that this requires a bi-directional port: you could use an open-collector, output-only pin to drive the data line for transmitting; and in input-only pin to sense the data line for receiving.