Typically you'd use a UserControl where what you want is a composite of
existing controls.  You'd create an instance of the UserControl either
by creating it standalone as a new item in your project, or by simply
dropping a new UserControl instance on your Form.

If you do the former method, it's similar to creating a Form-derived
class.  You get a whole new class, which you can then reuse in any
number of forms.  You can even put it in its own project and reuse it in
multiple projects.

Don't confuse a UserControl with a "custom Control", which is a
completely customized class derived from Control, in which you
completely define the behavior of the class.  A UserControl brings along
with it some default behavior that is especially useful when composing a
new control that itself contains several other controls.

Pete