But the 6 pin USB-TTL cable is a way of Serial Programming, and the 6 pin ICSP is a "In Circuit Serial Programming"......so how do they differ, excluding pin configuration? Undecided
The ICSP is the raw proramming interface which needs an external programmer which is how a raw AVR chip is programmed. The USB programming interface is what sets the arduino apart from a raw AVR. It provides a simple way for programming the AVR through a USB / serial interface and a bootloader program already programmed onto the ARduino chip.