A qubit exists in superposition α|0⟩ + β|1⟩ until measured. The Bloch sphere represents all pure qubit states geometrically — poles are |0⟩ and |1⟩, equator is equal superposition. Quantum gates are unitary rotations: H creates superposition (Hadamard), X flips (NOT), CNOT entangles two qubits. Entanglement links qubits non-locally — measuring one instantly determines the other. Quantum speedup exploits interference: constructive for correct answers, destructive for wrong ones. Shor's algorithm factors in polynomial time; Grover's searches √N steps.