Pin Description
RESET Used to reset the target system under the PC's control.
SCK The serial data clock
MOSI Master Out Slave In, used to send data from the PC (master) to the AVR (slave). Data is transmitted on the edges of SCK.
MISO Master In Slave Out, used to send data from the AVR (master) to the PC (slave). Data is transmitted on the edges of SCK.
LED The PC makes this line high to indicate that programming is occuring. Not used in all systems.
PWR Can be used either to provide power to the target system during programming, or for the programmer to draw power from the target system during programming. I use it to allow the programmer to draw power from the system.