Quantum gates and circuits form the fundamental architecture of quantum computing, analogous to logic gates and circuits in classical computers but operating on qubits rather than bits. Quantum gates are reversible unitary transformations represented as matrices that manipulate qubit states, preserving quantum information through their unitarity while enabling superposition and entanglement. Single-qubit gates like the Hadamard (H) gate create superposition from basis states, Pauli gates (X, Y, Z) perform bit flips and phase shifts, and phase gates (S, T) introduce relative phases essential for quantum interference. Multi-qubit gates such as the CNOT (Controlled-NOT) create entanglement by flipping a target qubit only when the control qubit is |1⟩, while Toffoli (CCNOT) gates enable universal quantum computation. A quantum circuit sequences these gates in a specific order, with wires representing qubits and measurement operations at the end collapsing superpositions into classical results. Together, they execute quantum algorithms like Shor's factoring algorithm and Grover's search, leveraging parallelism from N qubits processing 2^N states simultaneously to solve problems intractable for classical computers.