Boxing is the process of turning a value type object into a reference type object. Whenever a value type object is boxed, its value is copied into a new object on the heap and the reference to that object is stored. 
int number = 10;
 
// The following causes a box operation
// numRef will refer to an object on the 
// heap which contains a copy of number (10)
object numRef = number;
 
// Next unbox the value by casting it to 
// the appropriate type
int numberB = (int) numRef;
Unboxing is not type safe. It requires a cast because the compiler cannot determine the type of the object. If the object is not the correct type, it will cause an exception during the cast operation.
Large numbers of boxing and unboxing operations can become a performance problem. It should be avoided if possible. In particular, Generic types do not depend on boxing operations and they should be used whenever possible. Also, generic types maintain type safety.