Flip-flop circuits are designed to act as memory storage nodes.  They are able to maintain the value of a single binary bit, either 1 or 0.  Because they can hold the same electrical value for an extended amount of time, they are what are known as a bistable multivibrator.  This means that that there are two stable states, which the circuit can output, and those states depend on the current output and input of the device.

The most ubiquitous and simple flip-flops are called an SR latch.  This is made by simple routing signals appropriately through two different NOR gates.  These types of gates output a high value if both of the inputs are 0 and output a 0 if either or both of the inputs is 1.  The output of each gate is routed to one of the inputs of the other while the other inputs are called SET and RESET respectively.  In normal operation both the set and reset inputs are held low, which maintains the current state of the latch, whether low or high.  When SET is changed to high, the circuit output becomes a 1, and when RESET is changed to high, the circuit becomes a 0.

Flip-flops are used in the modern RAM memory packages.  They are a form of volatile memory, which means that an active power signal has to be maintained on the inputs for it to store a value within, a stable state.