# Difference between USART's synchronous and asynchronous mode

I'm writing software to initialise the EUSART1 module for RS232 on a PIC18F46K22. I'm using the usart.h library, built-in in the C18 compiler. There is some reference in MPLAB C18 C Compiler Libraries for the Open1USART() function on page 69 (73 in the PDF's numbering).

I now need to choose whether to use asynchronous or synchronous mode. What is the difference? Which should I choose when I just want to communicate with my pc, the common way (using RealTerm or so)?