The primary difference between classical and quantum computations lies in the way they process and store information. Classical computers use bits to represent information, which can be either a 0 or a 1. Quantum computers, on the other hand, use quantum bits or qubits, which can represent both 0 and 1 simultaneously, thanks to a quantum phenomenon called superposition.In terms of speed and complexity, quantum computers have the potential to perform certain complex calculations much faster than classical computers. This is due to another quantum property called entanglement, which allows qubits to be correlated with each other. When qubits are entangled, the information of one qubit can instantly affect the other, regardless of the distance between them. This enables quantum computers to perform multiple calculations simultaneously, leading to an exponential increase in processing power.Quantum computing utilizes quantum mechanics through superposition and entanglement to perform complex calculations that are impossible or would take an impractical amount of time for classical computers. For example, quantum computers can efficiently solve problems like factoring large numbers, simulating quantum systems, and optimizing complex systems, which are challenging tasks for classical computers.However, it's important to note that quantum computers are not meant to replace classical computers. They are designed to solve specific problems that are difficult for classical computers, while classical computers will still be more efficient for everyday tasks and general-purpose computing.