A composition is the framework for a movie. Each composition has its own timeline. A typical composition includes multiple layers that represent components such as video and audio footage items, animated text and vector graphics, still images, and lights. You add a footage item to a composition by creating a layer for which the footage item is the source. You then arrange layers within a composition in space and time, and composite using transparency features to determine which parts of underlying layers show through the layers stacked on top of them. (See Layers and properties and Transparency and compositing.)
A composition in After Effects is similar to a movie clip in Flash Professional or a sequence in Premiere Pro.
You render a composition to create the frames of a final output movie, which is encoded and exported to any number of formats. (See Basics of rendering and exporting.)
Simple projects may include only one composition; complex projects may include hundreds of compositions to organize large amounts of footage or many effects.
In some places in the After Effects user interface, composition is abbreviated as comp.
Each composition has an entry in the Project panel. Double-click a composition entry in the Project panel to open the composition in its own Timeline panel. To select a composition in the Project panel, right-click (Windows) or Control-click (Mac OS) in the Composition panel or Timeline panel for the composition and choose Reveal Composition In Project from the context menu.
Use the Composition panel to preview a composition and modify its contents manually. The Composition panel contains the composition frame and a pasteboard area outside the frame that you can use to move layers into and out of the composition frame. The offstage extents of layers—the portions not in the composition frame—are shown as rectangular outlines. Only the area inside the composition frame is rendered for previews and final output.
The composition frame in the Composition panel in After Effects is similar to the Stage in Flash Professional.
When working with a complex project, you may find it easiest to organize the project by nesting compositions—putting one or more compositions into another composition. You can create a composition from any number of layers by precomposing them. If you are finished modifying some layers of your composition, you can precompose those layers and then pre-render the precomposition, replacing it with a rendered movie. (See Precomposing, nesting, and pre-rendering.)
You can navigate within a hierarchy of nested compositions using the Composition Navigator and Composition Mini-Flowchart. (See Opening and navigating nested compositions.)
Use the Flowchart panel to see the structure of a complex composition or network of compositions.