Maxims MAX232 is one of those wonderful components that solves so many signal conversion problems.
This chip converts RS232 signal voltage levels to TTL voltage levels and visa-versa.
Hence if you need to communicate to your PC through it's serial port (COM1 or COM2) then this is the chip that can perform that function. If you have a microcontroller circuit, or a phone, or a calculator that requires a PC connection then this is the chip that is needed to make that communication happen.
For MAX232 data sheet, please click here
For MAX232 basics, please click here
For Maxim tutorials and application notes on using MAX232, click here