The container for all elements (compositions and/or footage files) is called the After Effects Project (aep.)

The "stage" for all footage files is called a Composition, or Comp for short.

Any basic element used within a project (images, videos, audio) is called a Footage File.

Create a new Composition by using the Ctrl-N shortcut. Introduce footage files native to AE by using the Layer>New menu. These options include the introduction of Text element, Shapes, and Null objects. We can also use this menu to introduce lights and cameras into a 3D scene.

A few things to keep in mind...

1) A project can (and often does) contain multiple compositions.

2) One composition can be nested within another composition.

3) A footage file can be used multiple times within a single composition.

4) One imported, a footage file can be used in any number of compositions within a project.

Let's look at how we can import elements from outside AE into our Composition...