It has 16 pins and the functionality of each pin is explained as follows:

Pin-1: It is the output 5. It goes high when the counter reads 5 counts.
Pin-2: It is the output 1. It goes high when the counter reads 0 counts.
Pin-3: It is the output 0. It goes high when the counter reads 0 counts.
Pin-4: It is the output 2. It goes high when the counter reads 2 counts.
Pin-5: It is the output 6. It goes high when the counter reads 6 counts.
Pin-6: It is the output 7. It goes high when the counter reads 7 counts.
Pin-7: It is the output 3. It goes high when the counter reads 3 counts.
Pin-8: It is the Ground pin which should be connected to a LOW voltage (0V).
Pin-9: It is the output 8. It goes high when the counter reads 8 counts.
Pin-10: It is the output 4. It goes high when the counter reads 4 counts.
Pin-11: It is the output 9. It goes high when the counter reads 9 counts.
Pin-12: This is divided by 10 output which is used to cascade the IC with another counter so as to enable counting greater than the range supported by a single IC 4017. By cascading with another 4017 IC, we can count up to 20 numbers. We can increase and increase the range of counting by cascading it with more and more IC 4017s. Each additional cascaded IC will increase the counting range by 10. However, it is not advisable to cascade more than 3 ICs as it may reduce the reliability of the count due to the occurrence glitches. If you need a counting range more than twenty or thirty, I advise you to go with conventional procedure of using a binary counter followed by a corresponding decoder.
Pin-13: This pin is the disable pin. In normal mode of operation, this is connected to ground or logic LOW voltage. If this pin is connected to logic HIGH voltage, then the circuit will stop receiving pulses and so it will not advance the count irrespective of number of pulses received from the clock.
Pin-14: This pin is the clock input. This is the pin from where we need to give the input clock pulses to the IC in order to advance the count. The count advances on the rising edge of the clock.
Pin-15: This is the reset pin which should be kept LOW for normal operation. If you need to reset the IC, then you can connect this pin to HIGH voltage.
Pin-16: This is the power supply (Vcc) pin. This should be given a HIGH voltage of 3V to 15V for the IC to function.