One of the parameters of the Win32 API function FindWindowEx is the Class Name of the window. For example, the Class Name of Microsoft Word is "OpusApp".
If I have developed my own application, what is going to be the Class Name of the windows of the app?
Can I set this Class Name to whatever I want?
You didn't notice when you created your window, that you had to call RegisterClassEx (or plain RegisterClass)? ;)
Each window has a class. When you create your own, you specify its class.
*Edit: given your ambiguous tagging, I'm not really sure how you wrote your app. If you're using .NET then you obviously didn't have to manually call the C++ function RegisterClass.*
Regardless, when created, each window is associated with a "class", which describes certain common properties for all windows belonging to that class.