flip flops RS(with NOR gates or NAND gates)
The idea of a flip-flop is a single input may change and the output does not. That's why you have two inputs.
The output is a voltage level. Inputs are sensitive to voltage levels. "Reading" means interpreting the voltage level as either "high" or "low".
The complemented output isn't a must. It however comes in very handy in many situations.