An XOR gate, or "exclusive OR" gate, is a digital logic gate that outputs true or high only when the number of true inputs is odd. In simpler terms, it gives a true output when one, and only one, of its two inputs is true. If both inputs are false or both are true, the output is false. XOR gates are commonly used in various applications, including computers, cryptography, and error detection. They play a crucial role in binary arithmetic and are essential for building more complex circuits, such as adders and multiplexers.