A counter is simply a device that counts up or down. Counters have various important applications. Counters may be used to count numbers, operations, quantities, or periods of time. They may also be used for dividing frequencies, for addressing information in storage, or for temporary storage. Counters are a series of flip-flops wired together to perform the type of counting desired. They will count up or down by ones, twos, or more. The total number of counts or stable states that a counter can indicate is called the modulus. For example, the modulus of a four-stage counter would be 1610, since it is capable of indicating (0000)2 to (1111)2. The term modulo is used to describe the count capability of counters. Modulo-16 represents a four-stage binary counter, modulo-11 represents a decade counter, modulo-8 represents a 3-bit binary counter, and so on.
The number of flip-flops used and how they are connected determine the number of states and the sequence of the states that the counter goes through in each complete cycle. Counters can be classified into two broad categories according to the way they are clocked: