Bloch sphere
The globe drawn to show what state a single qubit is in — north pole is zero, south pole is one, everything between is superposition.
A classical bit has two positions. A qubit is instead described by a point on a spherical surface, with two angles: one gives how probability is split between zero and one, the other gives the phase. Phase has no classical counterpart but is decisive — it is what makes interference possible, and therefore quantum computation itself.
Quantum gates become rotations of the sphere. A Hadamard gate turns the north pole out to the equator. The picture only works for one qubit; at two the geometry already fails, which is a useful reminder of how quickly intuition runs out.