Define classes and structure.

Class: User defined data type which contains data and methods to manipulate that data; is known as class. It is the fundamental packaging unit of OO technology. An object is a variable of a Class. Each object is associated with the data of type class with which it is created. Thus we can also say that class is a collection of objects of similar types. It is a user defined data type and behaves like built-in data type of the language. Since class has data and methods to manipulate the data, it supports one of the most important features of OO: Data Encapsulation.

Eg. 
class Student
{
           int rollno;
           int marks1, marks2;
           public:
                      void show(int r); // to print marks
                      void sum(int r); // to add the marks
};

We can create objects of class using:
class Student s1, s2;

Structure:
A structure is a collection of variables, referenced under one name, providing a convenient means of keeping related information together. Structure declaration forms a template which can be used to create structure objects. The variables inside a structure are called members. Generally all the members of a structure are logically related. Structure declaration precedes the keyword struct.

Consider following example:

struct address
{
            char name[80];
            char street[80];
            char city[20];
            char state[20];
};

Please note the semicolon at the end of structure declaration. This is done because a structure declaration is a statement. The type name of this structure is address. We can create structure variables using:

struct address ad1, ad2;
ad1 and ad2 are two variables of the type struct address..