They are instance of classes. It is a basic unit of a system. An object is an entity that has attributes, behavior, and identity. Attributes and behavior of an object are defined by the class definition.

A class acts as a blue-print that defines the properties, states, and behaviors that are common to a number of objects. An object is an instance of the class. For example, you have a class called Vehicle and Car is the object of that class. You can create any number of objects for the class named Vehicle, such as Van, Truck, and Auto.
The new operator is used to create an object of a class. When an object of a class is instantiated, the system allocates memory for every data member that is present in the class.

A collection can be defined as a group of related items that can be referred to as a single unit. The System.Collections namespace provides you with many classes and interfaces. Some of them are – ArrayList, List, Stack, ICollection, IEnumerable, and IDictionary. Generics provide the type-safety to your class at the compile time. While creating a data structure, you never need to specify the data type at the time of declaration. The System.Collections.Generic namespace contains all the generic collections.

All the methods inside an interface are always public, by default. You cannot specify any other access modifier for them.

Overriding involves the creation of two or more methods with the same name and same signature in different classes (one of them should be parent class and other should be child).
Overloading is a concept of using a method at different places with same name and different signatures within the same class.

Each delegate object holds reference to a single method. However, it is possible for a delegate object to hold references of and invoke multiple methods. Such delegate objects are called multicast delegates or combinable delegates.

The virtual keyword is used while defining a class to specify that the methods and the properties of that class can be overridden in derived classes.

Enumeration is defined as a value type that consists of a set of named values. These values are constants and are called enumerators. An enumeration type is declared using the enum keyword. Each enumerator in an enumeration is associated with an underlying type that is set, by default, on the enumerator. The following is an example that creates an enumeration to store different varieties of fruits:

enum Fruits {Mango, Apple, orange, Guava};
In the preceding example, an enumeration Fruits is created, where number 0 is associated with Mango, number 1 with Apple, number 2 with Orange, and number 3 with Guava. You can access the enumerators of an enumeration by these values.

Constructor is a special method of a class, which is called automatically when the instance of a class is created. It is created with the same name as the class and initializes all class members, whenever you access the class. The main features of a constructor are as follows:

• Constructors do not have any return type.
• Constructors can be overloaded.
• It is not mandatory to declare a constructor; it is invoked automatically by .NET Framework.

An interface is a template that contains only the signature of methods. The signature of a method consists of the numbers of parameters, the type of parameter (value, reference, or output), and the order of parameters. An interface has no implementation on its own because it contains only the definition of methods without any method body. An interface is defined using the interface keyword. Moreover, you cannot instantiate an interface. The various features of an interface are as follows:

• An interface is used to implement multiple inheritance in code. This feature of an interface is quite different from that of abstract classes because a class cannot derive the features of more than one class but can easily implement multiple interfaces.
• It defines a specific set of methods and their arguments.
• Variables in interface must be declared as public, static, and final while methods must be public and abstract.
• A class implementing an interface must implement all of its methods.
• An interface can derive from more than one interface.