The graphical user interface describes anything your application displays to the user. It is the primary way you interact with the user and allow him or her to interact with you. Creating great GUI's is something Visual Basic is amazing at. You can create professional interfaces with minimal effort. Learn how with these UI Visual Basic tutorials.
Graphical User Interfaces have been around for many years. As you develop more and more advanced applications it is worth your time to focus on your VB6 UI design. This includes understanding how each of the visual basic ui components work (such as buttons, textboxes, combo boxes, images, menus, etc.). This also includes understanding the different vb6 ui containers (such as Tabsets, Panels, status bars, grouping boxes, etc.) By understanding all of these components individually you will have a good knowledge of what can and can't be done.
From here you can focus on how your VB6 GUI fits together as a whole. For example what is the best way to layout a specific form using your individual input controls, labels, and grouping controls. Where should headings go? What order is the user most likely to want to enter their data? What should the tab order be? What controls should be shown vs collapsed? These are all questions a UI programmer needs to think through. Even though UI in Visual Basic is much easier than in many other languages, we as developers need to carefully think through what is most important to our end user and focus on these things above all else.