1.RS-232 is a standard for serial communication.
The standard include both protocol and interface.
'protocol' is the specification for software format.
(how the data is sent)
'interface ' is the specification of HW(like connectors , cables ...)
(about the voltage levels and which pin which signal...)
In practice ,we use both the terms .
The standard itself is very old , dating back to UNIX days.
But we make use of it in PCs also.
2. RS-232 , SPI and I2c all comes under serial form of data.
But the the 'protocol' part differs betwen them.
3. If you have two PCs connected by RS-232 connectors , MAX232 is not required.
MAX 232 translates the voltage levels alone.
5v (and 0v) to +12 and -12v and vice versa.
RS232 specifies +12 and -12 for data transfer.
But the microcontroller or any digital system operate only in 5v.
So we include an MAX232 between controller and RS232 interface.