Default type of a variable. Access Modifiers I mean.

public & private is not a type of variable..
it keywords to specify access levels for member variables. See This

And Your Answer is (access levels for member variables): Private

Let test this example...

class Test
    {
        String Test1;
        public string Test2;
        protected string Test3;
        private string Test4;
    }

    class TestAccessLavel
    {
        static void Main()
        {
            Test e = new Test();
            // e.Test1 Not acessable
            String Test2 = e.Test2;
            //e.Test3 Not acessable
            //e.Test4 Not acessable
        }
    }

Create one class Test and create variable (see above)
Now in other class main() create object of Test and try to acess Variable..

If Test1 & Test2 is acessable - public
Test1 & Test3 is acessable - protected
Test1 & Test4 is acessable - private

Thanks

In most cases the default access is private, including field declarations of the type you ask about.

The default access for a top level class, struct or interface, i.e. as in this example

namespace Test {
 class NoAccessModifier { }
}

... is internal. Note however that the default access for a nested class/struct/interface (i.e. one which is declared inside another class) is private, just like fields, methods etc defined inside a class).

If you are talking about string class then it is public in C#, or c++

Refer this
http://msdn.microsoft.com/en-us/library/system.string(v=vs.71).aspx[^]

and as already stated st is string type

http://www.codeproject.com/Articles/743113/Examples-on-access-modifiers-using-Csharp[^]

It is String only.
In the first case, you assigned "Hello" to it. While in second it is blank string "" by default. That's the only difference.

If you mean access specifier, then see the link.
http://msdn.microsoft.com/en-us/library/ba0a1yw2%28VS.80%29.aspx[^]
http://social.msdn.microsoft.com/Forums/en/csharpgeneral/thread/82a16a51-a6a6-41f0-89fb-66fd8876e06f[^]

string s;
is same as
internal string s;


Link:-http://msdn.microsoft.com/en-us/library/ms173121.aspx[^]

see the topic
Class and Struct Accessibility

Default access specifier is internal
Best of luck.........