Create Folders and Sub-Folders in SharePoint Programmatically

How to create Folders in SharePoint 2013?
To create a folder in SharePoint list or Library, follow these steps:

• Navigate to your SharePoint list or library
• From the "Files" tab on the Ribbon, Click on "New Folder" button
• Give a name to your new folder and click "Save"

What if you don't find "New Folder" button in the ribbon? 
If you don't get "New Folder" button, it could be disabled. Here is how to enable folders in SharePoint list:

• Go to List Settings, Click on "Advanced Settings"
• Scroll down and set "Yes" to Make "New Folder" command available option.

In a Migration automation tool development, had to create folders and sub-folders from the code programmatically. Here I'm sharing the code snippets in C#

  
using (SPSite site = new SPSite("http://sharepoint.company.com"))
{
    using (SPWeb web = site.OpenWeb())
    {
         //Get the List
         SPList list = web.Lists["Project Tracker"];

         //Create a Folder
         SPListItem folder = list.AddItem(list.RootFolder.ServerRelativeUrl, SPFileSystemObjectType.Folder, "Sales Documents");
         folder.Update();

         //Create a Sub-Folder
         SPListItem SubFolder = list.AddItem(folder.Folder.ServerRelativeUrl , SPFileSystemObjectType.Folder, "APAC Sales Docs");
         SubFolder.Update();

     }         
}

Alternatively, you can use:

//Get the List
SPList list = web.Lists["Shared Documents"];
String url = list.RootFolder.ServerRelativeUrl.ToString();
SPFolderCollection folders = web.GetFolder(url).SubFolders;

//Create new folder
folders.Add("New Documents");

Here is the PowerShell to create a folder in SharePoint: How to Create a Folder in SharePoint Document Library using PowerShell?