Source book: Quantum Computation and Quantum Information, Michael A. Neilsen and Isaac L. Chuang
Analogy between classical bit and quantum bit (qubit).
Classical bit has 0 or 1 value whereas qubit can have state $|0\rangle$ or $|1\rangle$ along with linear combination ot those states (superposition).
While classical bit can be measured deterministically with value either 0 or 1, qubit when measured will result in 0 or 1 with probability of $|\alpha|^2$ and $|\beta|^2$ respectively.
Example for measurment of $|+\rangle|$ state.
Qubits can be realized with physical phenomenon. For example, light can be shined on atom with appropriate energy and time to move quantums states.
Geometric representation of qubit is possible through Bloch Sphere.
Whenever, qubit is measurement, its state collapses.
