Module 5.1 deals with clock oscillators, which are basically types of square wave generators or oscillators that produce a continuous stream of square waves or a continuous train of pulses (a "square" wave whose mark to space ratio is NOT 1:1). These pulses are used to sequence the actions of other devices in the sequential logic circuit so that all the actions taking place in the circuit are properly synchronised.
Bi-stable devices (popularly called Flip-flops) described in Modules 5.2 to 5.4, are sub-circuits, usually contained within ICs, and are the most basic type of 1-bit memory. They have outputs that can take up one of two stable states, Logic 1 or logic 0 or off. Once the device is triggered into one of these two states by an external input pulse, the output remains in that state until another pulse is used to reverse that state, so that a logic 1 output becomes logic 0 or vice versa. Again the circuit remains stable in this state until an input signal is used to reverse the output state. Hence the circuit is said to have Bi (two) stable output
Various types of digital counters are described in Module 5.6. Consisting of arrangements of bi-stables, they are very widely used in many types of digital systems from computer arithmetic to TV screens, as well as many digital timing and measurement devices.