S-R Flip-Flop

A set-reset flip-flop can be constructed from cross-coupled NOR gates.
The set and reset inputs of the cross-couples NOR flip-flop are high active.
A set input will force the Q output high and the output low.
A reset input will force the Q output low and the output high.
With neither the set input nor the reset input active, the flip-flop is in a "hold" condition and will "remember" the last input condition.

A S-R flip-flop can also be made using cross-coupled NAND gates.
The cross-coupled NAND flip-flop has low active set and reset inputs. These can be made high active by using inverters.
A flip-flop is "latched" when the Q output hold the last input condition.
A flip-flop is said to be transparent when the Q output responds immediately to a change on the input.
A register is a group of flip-flops used to store a binary word. One flip-flop is needed for each bit in the data word.
AND gates can be used to "strobe" or enable data gated into a register.