Where to Use 74HC595 IC
The 74HC595 is an 8-bit Serial In – Parallel Out Shift Register, i.e. it can receive (input) data serially and control 8 output pins in parallel. This comes in very handy where do not have enough GPIO pins on our MCU/MPU to control the required number of outputs. It is often used in projects where relatively a large number of LED’s has to be controlled through the Microcontroller. It can also be used to interface LCD screen since it can act as the data bit for the LCD displays. It can also be used to control 5V loads like relays through a 3.3V microcontroller since the high-level voltage is only 3.15. So if you are looking for an IC to save on your GPIO pins on the Microcontroller then this IC might be the right choice for you.
 
How to Use a 74HC595 IC
The 74HC595 shift register is commonly used with microcontrollers or microprocessors to expand the GIPO functionalities. It requires only 3 pins connected to the MCU, which are Clock, Data and Latch. It has a wide operating voltage from 2V to 6V.
The pins 11, 14 and 12 are connected to the GPIO pins of the Microcontroller. In which pin 11 is the clock which sends a constant pulse to keep timing. The pin 14 is Data which actually sends the Data about which output pins has to stay low and which should go high. The Pin 12 is the Latch which updates the received  data to the output pins when made high, this pin can also be permanently held high. 
Features
8-bit, Serial In – Parallel out Shift register
Operating Voltage: 2V to 6V
Power Consumption: 80uA
Output source/sink current: 35mA
Output Voltage is equal to Operating voltage
Minimum high-level Input Voltage: 3.15V @(Vcc=4.5V)
Maximum low-level Input Voltage: 1.35V @(Vcc=4.5V)
Can be easily cascaded with more IC to get more outputs
Maximum Clock Frequency: 25Mhz @4.5V
Available in 16-pin PDIP, GDIP, PDSO packages