Latches were level sensitive; means the state of latch gets change according the values of input signal only when the clock is active. But practically there is often need to have a storage elements that can change their state only once during a clock cycle. Flipflop fall under this category. Master Slave and D Flipflop are the two basic types of flipflops.
As seen in the figure, Master-Slave D Flip-flop consists of two D Latches. Master changes its state when clock=1 while the latter changes its state when clock=0. When the clock is high the masters track the value of D but since the slave is in inactive state, Qs also remains unchanged, Now when the clock signal goes low, the master goes to inactive state and the slave which is now in active state it tracks the value of Qm. While clock=0, Qm does not change its value. Thus we can say that only once during the clock cycle the slave can undergo change in its value. This gives us a generic idea of working of flip-flops.
It can also be observed that only during the transition from 1 to 0, the output gets change. This transition is referred to as "an edge". Transition from 1-0 is termed as negative edge while in the opposite case it is called positive edge. In this particular case our flip-flop is negative edge triggered. Bubble with arrow indicates that the flipflop is negative edge triggered.
More to follow