Sequential circuits consist of combinational logic with feedback provided by storage elements like latches and flip-flops. There are two types of sequential circuits: asynchronous which depend on input signal order and are difficult to design, and synchronous which are defined by discrete clock pulses and are easier to design. Common storage elements include SR latches, D latches, and edge-triggered master-slave flip-flops which are used to build synchronous sequential circuits like RAM, ROM and other digital devices that can store a single bit.