Object-Oriented Programming is a programming paradigm based on the concept of 'objects', which can contain data in the form of fields and code in the form of procedures known as methods. It enables modularity, code reuse, and abstraction.

The four basic principles of OOP are Encapsulation, Abstraction, Inheritance, and Polymorphism. Encapsulation hides the internal state of the object; Abstraction simplifies complex reality by modeling classes appropriate to the problem; Inheritance allows a class to be defined based on another class; and Polymorphism allows for methods to do different things based on the object it is acting upon.

A class is a blueprint or prototype from which objects are created. It defines a datatype by bundling data and methods that work on the data into one single unit. An object, on the other hand, is an instance of a class. When a class is defined, no memory is allocated until an object of that class is created.

Constructors are special class functions which perform initialization of every object. The constructor method in a class initializes an object immediately upon creation. Typically, it's used to set initial values for attributes and perform any setup steps that are required when an object is created.

Method overloading is a feature that allows a class to have more than one method having the same name, if their parameter lists are different. Method overriding, on the other hand, occurs when a subclass provides a specific implementation for a method that is already defined in its superclass. The subclass overrides the method in its parent class.

Access specifiers define the scope of a member (variable or method) in a class. The most common access specifiers are: Public (accessible from any other class), Private (accessible only within the class they are defined), and Protected (accessible within the class they are defined and in its subclass).