Forms are used to view and input data. They comprise of a set of controls, which can be bound to a table's field or used by a script.
Forms are created and modified in Form Design view. You can add controls, set up sizing, set control attributes, set data bindings and set script actions in form design view.
You can read more about form design view here.
Once the structure of a form is set out, you employ Form Data view to use it. You can use the controls set up in form data view to modify data, execute scripts, etc. You can also add and remove rows, and insert rows into relationships.
You can read more about form data view here.