What is differance between int _a and int a

Use of a leading underscore is a convention in C and C++ to indicate something that the compiler uses. There is nothing in the C and C++ languages that prevents anyone from using a leading underscore (such as in "_a") but the compiler developers suggest not to. It is not uncommon however for programmers to ignore that advice.

So why am I talking about C and C++ when your question is about C#? Because the C# language is very much like C++. There are many variations used for making a variable's name unique; one way is to use the leading underscore. There is absolutely nothing that requires use of a leading underscore but it is one of very many alternatives that some developers use. In C++ Microsoft uses "m_" as the prefix for members of a class. Some developers might use "my". I wish someone would design a better way of doing it but the syntax of a solution is not simple and obvious.

Properties are a way of shielding data from code outside the class.

The data cannot be accessed directly but only through the property which can make sure that you don't assign values which are invalid, carry out subsidiary calculations and so forth.

Suppose, for example that you wanted '_a' to only be assigned values of between 0 and 9 inclusive. You could achieve that with the following property:

class MyClass
{
   int _a;

   public int A
   {
       get { return _a; }
       set
       { 
          if (value < 0 || value > 9) throw new ArgumentException("Value must be between 0 and 9");
          _a = value;
       } 
   }
}

For this reason, properties are sometimes called 'smart fields'.

In C# it's possible for a variable to begin with an underscore as well as a letter.

So _a and a are different variables.

It's common when declaring private fields in a class to precede the field name with an underscore and then expose it via a public property which begins with an uppercase letter. For example:

class MyClass
{
   int _a;

   public int A
   {
       get {return _a; }
       set { _a = value; }
    }
}