Unfortunately there is no really good term. RS-232 is incorrectly used often to describe this interface I guess UART is about as good as we have. As far as the payload, the data itself, one symbol is one bit.
You can simply look up UART at wikipedia.
As far as the TTL levels go the line is idle high. A start bit is one bit period long and is low, telling the receiver some data is coming, the next some number of bits agreed upon both sides, say 7 or 8 bits is very common come next least significant bit first. One bit cell per bit. So if you want to send the byte 0x53 the next bits would be 11001010. And then at least one stop bit, high or sometimes two. Start, payload, stop. If you didnt have another character to send then you just left the line high.
Other options are parity, even, odd, or non (no bit sent). So 8N1 is pretty common, 8 bits no parity one stop bit. Parity is computed across the payload, the data bits, not the start and stop.
In order to send it from computer to computer that is where RS-232 comes in it defines the voltages levels and pins, a one is something less than -3Volts to -15Volts a zero is +3 volts to +15. You take the TTL level signals and send them through a transceiver and then into a common connector to make it a COM or SERIAL port on a PC for example.
Baud and bitrate are related if you are using 9600 baud it is 9600Hz, each bit cell is 1/9600 or 0.000104...seconds or 9600 bit cells per second, but they didnt want to say bits per second, because one it is not necesarily continous and you have some extra bit cells the start, parity and stop bits that are not data bits. So at 9600 baud if you were running 8N1 that is 10 bit cells per character or byte if going as fast as possible or 960 chracters per second, 7680 data bits per second. 9600 8E1 is different though a little slower 11 bits cells per character not 10.
RS-232C, RS-485, RS-422 are different electrical standards for describing the voltage levels on the bits as well as either connector or pins or other, for example using a differential pair, one side of the pair is the low voltage for a one while the other signal in that pair would be the opposite. Can get cleaner/longer runs that way, the receiver measures the difference between them one relative to the other rather than one signal relative to a common ground, to extract the one or zero state. RS-422 can send uart data as can RS-232 it doesnt define the start/stop/data nature of the bit stream it just describes the voltage levels pins and wiring.
When you take an arduino or a raspberry pi for example, these have uarts, but they are not RS-232 you would destroy the chips if you hooked them up to RS-232, they are ttl level 5.0V or 3.3v with a zero being ground and a one being high, you can for a couple bucks or less buy usb to uart devices based on FTDI chips and others that can be treated the same way as a com/serial port on an older computer was, windows, linux, etc see them as com/tty ports and you can use the same software but they are not RS-232 they are just TTL level uart and you can wire them directly to your arduino or raspberry or other microcontroller or processor if you use the right voltage levels and are safe with your ground reference.
