Quantum Superposition
Superposition lets a qubit hold a weighted combination of |0> and |1> at once; interference over these amplitudes is where quantum algorithms get their power.
More than a coin
A classical bit is 0 or 1. A qubit can be a superposition alpha|0> + beta|1>, with complex amplitudes constrained by |alpha|^2 + |beta|^2 = 1. It is tempting to say the qubit is 'both at once', but the useful content is subtler: the amplitudes carry relative phase and can interfere.
Measurement collapses it
Measuring in the computational basis returns 0 with probability |alpha|^2 and 1 with probability |beta|^2 (the Born rule), and the superposition collapses. You never read out alpha and beta directly -- only sampled outcomes. The Bloch sphere gives the geometric picture of a single qubit's state.
Why it powers algorithms
Apply a Hadamard to each of n qubits and you get an equal superposition of all 2^n basis states -- quantum parallelism. But parallelism alone is not a speedup: measurement yields just one bitstring. The art of a quantum algorithm is arranging interference so that amplitude concentrates on the answer before you measure.