At microcontroller level, the signal is called UART and usually idles high. What is called RS232 is after the level converter like the MAX232, where the levels are around +-12V.
About the signal format, it idles high and a start bit, what is needed to identify the start of the frame, is the first High to Low transition, the decoder needs to discard this start bit and then start measuring high and low values until the stop bit (low to high transition). It is an asynchronous protocol, it is very important to set the parameters correctly: baud, number of bits, idle high or low, parity, stop bits, etc. As it is just a streaming of level transitions, the 2 devices need to know exactly how the information is encoded.