Uart
Universal Asynchronous Transmitter. It's part of your computer's hardware -- the circuit inside your serial port where you connect your modem. It takes the information from your computer (which is parallel) and converts it into a form recognisable by your modem (which is serial).

Essentially, the UART acts like a canal, balancing the flow of information to ensure you don't get too much coming in or going out at once. If you have an older computer with slow UART settings, and you've added a 56k modem, the UART could get overloaded, so it's well worth upgrading. (Also see CMOS)