A Layout defines the general look and feel of the Object and determines what actions can be taken for the Object.
There are currently four basic layouts that users can choose for their Objects, and there are plans for contributors to be able to create new layouts in the future.
You can find the object layout menu above the Object title; next to the cover and icon buttons. This is where you can change the layout of the object or set the layout width. Some layouts even let you change the text alignment.