Objects are instances of classes, and they encapsulate both data (attributes) and the methods (functions) that operate on that data. Classes are a fundamental concept in object-oriented programming (OOP) that enable you to create user-defined data types.

Here are some basic concepts about OOP classes:

1. Abstraction: You can abstract complicated data structures and operations using classes. Without disclosing the internal implementation details, you can create a class to represent a real-world entity and provide an easy way to interact with it.
2. Encapsulation: In a single unit (the class), data (attributes) and methods (functions) that operate on those attributes are combined. Encapsulation ensures that operations are carried out consistently and helps in restricting access to internal data.
3. Inheritance: You can build a new class (subclass or derived class) from an existing class (base class or parent class) using inheritance. The base class's attributes and methods are inherited by the subclass, which can also add new attributes and methods or change those that are already present.
4. Polymorphism: Objects of various classes can be treated as belonging to a single superclass by using  polymorphism. As a result, you can create code that interacts with objects belonging to different classes using a common interface.
5. Instance and Class Variables: Instance variables are unique to each instance of a class, whereas class variables are shared by all instances of the class. The attributes that are unique to an object are stored in instance variables, whereas the attributes that are shared by all instances are stored in class variables.
6. Constructor ('__init__' method): When a new instance of the class is created, the constructor, a special method in the class, is called. The object's attributes are initialized.
7. Method Overriding: You can modify (override) base class methods in a subclass by using inheritance. This is helpful if you want to offer a specialized implementation of a method in the subclass.
8. Access Modifiers: Access modifiers are available in some programming languages to regulate the visibility of class members (attributes and methods). If a member can be accessed from outside the class is determined by these modifiers.
9. Composition: This involves assembling complex objects from simpler ones. You can create more intricate data structures by using a class's ability to have attributes that are instances of other classes.

For working with classes, each programming language has its own syntax and conventions. Here is a simple Java example:

#include <iostream>
using namespace std;
class Rectangle {
private:
    double width;
    double height;

public:
    Rectangle(double w, double h) : width(w), height(h) {}        //  calculate the area

    double calculateArea() {
        return width * height;                                      //  calculate the area
    }

    double calculatePerimeter() {
        return 2 * (width + height);                                 // calculate the perimeter
    }
};

int main() {

    Rectangle myRectangle(7.0, 4.0);                         // Creating a rectangle object with a width 7 and height 4

 
    double area = myRectangle.calculateArea();                          // Using class methods to perform calculations
    double perimeter = myRectangle.calculatePerimeter();

    // Display the results
    cout << "Rectangle Area: " << area << endl;
    cout << "Rectangle Perimeter: " << perimeter << endl;

    return 0;
}