simple about c.........

I think you need to take one step back, and get clear about what the difference is between Value types and Reference Types in .NET, and how they are stored in memory as they are created and used.

You "declare" variables in your code: they may or may not be automatically 'initialized' to some default value, depending on several factors (which you can understand by getting clear about Value and Reference Types).

I find the use of the word "defined" a little ambiguous because: at times it's used to mean the same thing as declared, and at other times it's used to mean the variable has a value assigned to it, or the (reference Type) variable has been instantiated

It may be helpful to think that you "assign values" to Value type variables, but you "create instances" of reference types. In the case of the 'string' type it gets a bit complex, and there's a good discussion of that complexity here[^].

Get a good book on C#, perhaps one from Jesse Liberty, and get "grounded" on what .NET Types are, and what variables are; from there you can consider the use of 'static' vs. non-static Types, and so forth.

A little study now, and you can solve so many problems later.

best, Bill

There is big deference between declaring and defining a variable

//when you write this then it means declaring a variable
int a;

//or 
float b;


// when you assign any value to that variable is called defining

a = 10;
//Or
b =22.56;
// you can also write this type of statements its called defining at declaration time
int a = 10;
//or 
float b = 22.56;
//or 
float c = a/b;

Very basic.
"Declare" as word state, we announce a veriable/function with it's datatype

int a;

Defination means to assign/describe veriables/function

a = 10;

Declaring a variable is just like

string str;

defining means assigning a value to that variable as below

str="Hello";

After declaring a variable if we assign a value then memory is allocated to that variable in stack, this is called initializing a variable.

A definition gives information about the type of a variable, structure or class, such as:

class MyClass
{
public:
    MyClass();
    func(int i, string& str);
;

MyClass is defined as a class with a constructor and a single method; the implementation will be in a source code file (.cpp).
You declare an object when you create it by declaring its type and scope, thus:

MyClass myObject;

myObject is declared as an object variable of the MyClass class. A very simple example would be the case where an external variable is defined in a header file but declared in a source, thus:

// prog.h
extern int GlobalValue;    // the definition, GlobalValue is a global
                           // integer, but it does not exist yet
...

// prog.cpp
int GlobalValue = 0;   // the declaration, GlobalValue is declared in
                       // this module with a value of zero
...