Difference bet instantinstantiation of class and declaring object of class

In the first case you have not intantiated the object. As a result, you only have access to the classes static methods.

Ine the second case, you are actually allocating a location for an instance in memory. You are actually creating an object of that class. This is what you would do in OOPS. Create objects and then set properties for each of these objets.

There is tons of information about this on the internet.
Something like this[^] should be good reading for you.

when you declare a object like

classA objA;

then object of a class is created with null reference

when you instantiate an object like

classA objA = new classA()

then all the variables and methods of class is referenced to a object.

Instantiation: the creation of an object (allocating memory for it).

declaration or initialisation: setting the default values.

public class A
{
    int field1;
    public A()
    {
        // Start Initialization.
        field1 = 2;
        // End Initialization.
    }
    public void MyInit()
    {
        // Start Initialization.
        field1 = 1;
        // End Initialization.
    }
}

protected void Page_Load(object sender, EventArgs e)
{
    A a; // Object declaration

    // A "new" keyword means allocating memory for it 
    a = new A(); // Object Instantiation and Initialization

    // Another Initialization
    a.MyInit(); // Initialized by calling another method
}