Category

Element selectedElement = null;
foreach (ElementId id in uidoc.Selection.GetElementIds())
{
    selectedElement = document.GetElement(id);
    break;  // just get one selected element
}

// Get the category instance from the Category property
Category category = selectedElement.Category;

BuiltInCategory enumCategory = category.BuiltInCategory;

// Format the prompt string, which contains the category information
String prompt = "The category information of the selected element is: ";
prompt += "\n\tName:\t" + category.Name;   // Name information

prompt += "\n\tId:\t" + enumCategory.ToString();    // Id information
prompt += "\n\tParent:\t";
if (null == category.Parent)
{
    prompt += "No Parent Category";   // Parent information, it may be null
}
else
{
    prompt += category.Parent.Name;
}

prompt += "\n\tSubCategories:"; // SubCategories information, 
CategoryNameMap subCategories = category.SubCategories;
if (null == subCategories || 0 == subCategories.Size) // It may be null or has no item in it
{
    prompt += "No SubCategories;";
}
else
{
    foreach (Category ii in subCategories)
    {
        prompt += "\n\t\t" + ii.Name;
    }
}

// Give the user some information
TaskDialog.Show("Revit",prompt);