With serial communication you can exchange data with the AVR-microcontroller and your PC. Allmost all AVR-microcontroller have a UART (Universal Asynchronous Reciever/Transmitter) on board of the chip (accept for some ATTiny microcontrollers).

short summary of C-programming language for AVR microcontrollers

WinAVR is a set of open source software development tools for C programming the AVR microcontroller family. It includes the GNU GCC compiler for C and the Programmers Notepad.
To learn programming in C you have to install WinAVR first. This small project let you make running leds. You need a attiny2312 microcontroller, eight LED's and 8 resistors of 150ohm.