Layers in AE

You can create several kinds of layers:

• Video and audio layers that are based on footage items that you import, such as still images, movies, and audio tracks
• Layers that you create within After Effects to perform special functions, such as cameras, lights, adjustment layers, and null objects
• Solid-color layers that are based on solid-color footage items that you create within After Effects
• Synthetic layers that hold visual elements that you create within After Effects, such as shape layers and text layers
• Precomposition layers, which use compositions as their source footage items

SOURCE:
http://help.adobe.com/en_US/AfterEffects/9.0/WS3878526689cb91655866c1103906c6dea-7e9da.html