terryking228:
74HC595 Serial-In-Parallel-Out Expander This IC is a Shift Register so that 8 bits of data can be 'shifted' into it over just 2 wires. This is often used to add more functional Digital Outputs to Arduino. It is most often controlled by the Arduino standard function shiftOut. See the Reference Page: http://arduino.cc/en/Reference/ShiftOut
You can control a 595 with two wires only if you connect the shift register and storage register clocks together. I wouldn't generally recommend that because it exposes the transient and incorrect data patterns being shifted in to whatever device is connected to the shift register and that can cause glitches or other unexpected behavior. Shift in all your bits, then latch them into the storage register. Three wires required for that.