Sequential circuits are those whose outputs are not only dependent on the inputs but also on the sequence in which inputs are applied.
Sequential circuits require that their outputs or states be stored for use in generation of further outputs. This means that sequential circuits use memories for data storage. Data here, is in form of bits. To store and retrieve bits, semiconductor memory elements, called latch and flip-flop, are used.
Basic sequential circuit application could be MEMORY. A group of bit-storage elements that act as semiconductor memory, is called Register. Next simple application is counting certain events with use of counters.