The USB to TTL cable has a 4-terminal connector: +5v, Tx, Rx and Gnd. The +5v is a supply voltage and is only required if the circuit is to be powered from the USB cable. The +5v (yellow) can source up to 100mA - which is sufficient to power a circuit with a microcontroller and a few LEDs etc. The Tx (blue) is the transmit wire with respect to the TTL circuit, that is data sent from the microcontroller the PC. The Rx (red) is the receive wire with respect to the TTL circuit, that is data sent from the PC to the microcontroller. The Gnd (black) wire is the Ground (0v) connection to reference the voltage to. Figure 1 shows a typical circuit connection. Figure 1 shows a PIC 16F627 microcontroller with connections for the USB to TTL cable [the 16F627 is currently the smallest PIC microcontroller with built in UART peripheral].
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