I have been to an interview and was asked this question - is there any differenc

Question

I have been to an interview and was asked this question - is there any difference adding or removing the static keyword in these classes?

I know what static means but my understanding of this point is weak.

class Program
{
    static void Main(string[] args)
    {
            aAsposeNew.MultipleInstancesProblem();
            Console.ReadLine();
    }
}

public static class aAsposeNew
{
    public static void MultipleInstancesProblem()
    {
            var task1 = Task.Run(() => CreateDocument("document1.pdf"));
            var task2 = Task.Run(() => CreateDocument("document2.pdf"));
            var task3 = Task.Run(() => CreateDocument("document3.pdf"));
    }

    public static void CreateDocument(string documentName)
    {
        var doc = new Document();
        doc.Pages.Add();

        var table = new Aspose.Pdf.Table();
        var row = table.Rows.Add();
        table.ColumnWidths = "600";
        var hf =
            new HtmlFragment(@"<ul>
                                 <li>Internal HR Meeting Outcome</li>
                                 <li>Internal HR Meeting Outcome January 2015</li>
                               </ul>");
        var cell = row.Cells.Add();
        cell.Paragraphs.Add(hf);
        doc.Pages[1].Paragraphs.Add(table);

        doc.Save(@"C:Output" + documentName);
    }
}
and without the static keyword:

class Program
{
    static void Main(string[] args)
    {
            aAsposeNew aAN = new aAsposeNew();
            aAN.MultipleInstancesProblem();
            Console.ReadLine();
    }
}

public class aAsposeNew
{
    public void MultipleInstancesProblem()
    {
            var task1 = Task.Run(() => CreateDocument("document1.pdf"));
            var task2 = Task.Run(() => CreateDocument("document2.pdf"));
            var task3 = Task.Run(() => CreateDocument("document3.pdf"));
    }

    public void CreateDocument(string documentName)
    {
        var doc = new Document();
        doc.Pages.Add();

        var table = new Aspose.Pdf.Table();
        var row = table.Rows.Add();
        table.ColumnWidths = "600";
        var hf =
            new HtmlFragment(@"<ul>
                                 <li>Internal HR Meeting Outcome</li>
                                 <li>Internal HR Meeting Outcome January 2015</li>
                               </ul>");
        var cell = row.Cells.Add();
        cell.Paragraphs.Add(hf);
        doc.Pages[1].Paragraphs.Add(table);

        doc.Save(@"C:Output" + documentName);
    }
}

Explanation / Answer

In addition to the two options you posted (static method in static class, instance method in non-static class), there is a third option: a static method in a non-static class:

class Program
{
static void Main(string[] args)
{
aAsposeNew.MultipleInstancesProblem();
Console.ReadLine();
}
}

public class aAsposeNew
{
public static void MultipleInstancesProblem()
{
// elided
}
}
I distinguish between the cases as if the class had some instance fields (or auto-properties), there would be a difference, as there'd be a memory impact from instantiating the class with new that you wouldn't get if it were a static method. For your code this doesn't matter as there are no instance fields or auto-properties.

This static method in non-static class case is still the same as the method being static in a static class, which, for your case, is the same as an instance method in an instance class (other than the way the method is called as @craysiii points out in his answer.

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