Serial RS-232 (V.24) communication works with voltages (between -15V … -3V are used to transmit a binary ‘1’ and +3V … +15V to transmit a binary ‘0’) which are not compatible with today’s computer logic voltages. On the other hand, classic TTL computer logic operates between 0V … +5V (roughly 0V … +0.8V referred to as low for binary ‘0’, +2V … +5V for high binary ‘1’ ). Modern low-power logic operates in the range of 0V … +3.3V or even lower.
So, the maximum RS-232 signal levels are far too high for today’s computer logic electronics, and the negative RS-232 voltage can’t be grokked at all by the computer logic. Therefore, to receive serial data from an RS-232 interface the voltage has to be reduced, and the 0 and 1 voltage levels inverted. In the other direction (sending data from some logic over RS-232) the low logic voltage has to be “bumped up”, and a negative voltage has to be generated, too.
All this can be done with conventional analog electronics, e.g. a particular power supply and a couple of transistors or the once popular 1488 (transmitter) and 1489 (receiver) ICs. However, since more than a decade it has become standard in amateur electronics to do the necessary signal level conversion with an integrated circuit (IC) from the MAX232 family (typically a MAX232A or some clone). In fact, it is hard to find some RS-232 circuitry in amateur electronics without a MAX232A or some clone. The MAX232 is quite cheap (less than 5 Euros) or if your lucky you can get a free sample from Maxim.
The MAX232 from Maxim was the first IC which in one package contains the necessary drivers and receivers to adapt the RS-232 signal voltage levels to TTL logic. It became popular, because it just needs one voltage (+5V or +3.3V) and generates the necessary RS-232 voltage levels.
Source: TomicIgor (elektronika.ba)