The circuit is based around the 4518 counter IC. It is a CMOS type and can work from a wide voltage range. The circuit shown will count input pulses. Leave inputs as they are if not using them.
It is a digital device and works with logic level inputs. A high refers to +v and low refers to 0v.
The outputs A,B,C,D are binary signals, A is the LSB (Least Significant Bit), ie = 1
The output increments its value on every rising clock input. As it is a BCD counter, it counts from 0000 to 1001 (not 1111) and represents a decimal place..
More places/counters can be added in the same way following these rules:
An unlimited number of places can be added to count to very high numbers.