A serial interface is a simple way to connect an FPGA to a PC.
This project shows how to create an asynchronous serial link like RS-232 in an FPGA.
It creates a signal "TxD" by serializing the data to transmit.
It takes a signal "RxD" from outside the FPGA and "de-serializes" it for easy use inside the FPGA.
The project's code can be found here.