Desktop applications are present for almost every purpose. Some times, it is even difficult to imagine computers without some of the desktop applications, that we use on almost daily basis. 
On desktop applications, you have simple applications like calculator or very complex application like Microsoft Office. You can have complex 3-D designing software, Software engineering tools, Scientific tools everything as Desktop Application. These applications are very different from each other, and it is very interesting to know about complexity and challenges involved in these applications.