A composition is a container for your visual elements. Whenever you want to create a visual effect in Adobe After Effects, you will have to create a composition first. You then add your visual elements, be they images or video footage, as layers into this composition.
You can then modify the layers, blend them together and apply all sorts of effects before exporting your composition into a final video
You can also use compositions to group together a number of layers. You do this by pre-composing a number of layers and nesting them in a new composition. This new composition then replaces the original layers.
The advantage of this grouping is that you can now deal with all the layers you pre-composed as a single layer and move them around or apply effects to the entire group.
Finally - to me the most powerful aspect of compositions - is that you can use compositions to create reusable elements for your effects. You add a number of visual elements into a composition and then you can use copies of that composition as often as you want.
If you now change the contents of this composition, you will affect all instances of it across all of your layers!
I am going to use 2 pieces of footage for this tutorial:
Here's a peek at the explosion stock footage we will be using