Forms serve a variety of functions with various designs.
Although most frequently used to enter or view data, forms have several other valuable uses. Since form follows function, each of these uses has its own type of form.
You can create a form that appears as a switchboard when a user opens a particular database. The switchboard provides a friendly and controlled way to open forms, reports, and other objects. It guides users to the actions that you want them to perform, and screens parts of your database that you don't want people messing with.
Forms can display messages about a database. A form can provide instructions for using the database, or assistance for troubleshooting any problems.
A subform looks like a part of another form. In fact, while the larger main form connects to one table, the subform connects to a different table. A user can enter data into one form and view related data in the other, without ever knowing that they are actually separate and equal.