A copy constructor is a special type of constructor in programming, particularly in languages like C++, that initializes a new object as a copy of an existing object. It takes a reference to an object of the same class as its parameter and creates a new instance with the same values for its member variables. Copy constructors are essential for managing resources, such as dynamic memory, when objects are copied. If a class does not define a copy constructor, the compiler provides a default one, which performs a shallow copy. However, for classes that manage resources, a custom copy constructor is often necessary to ensure proper resource management and avoid issues like double deletion.