J-K flip-flop can be considered as a modification of the S-R flip-flop.
The main difference is that the intermediate state is more refined and precise than that of an S-R flip-flop.
The characteristics of inputs 'J' and 'K' is same as the 'S' and 'R' inputs of the S-R flip-flop.
J stands for SET, and 'K' stands for CLEAR.
When both the inputs J and K have a HIGH state, the flip-flop switches to the complement state, so, for a value of Q = 1, it switches to Q=0, and for a value of Q = 0, it switches to Q=1.