An object can be Text, Shapes, Images or anything that occupies a bound box.
A bound box is the box you see when you choose the Move Tool.
This generally works on two or more objects.
First open your Layers Pallet.
Next link the layers you want to align (*note* the chain links in the above image show that Image and Text us linked to Shape).
Pick the object you want as the anchor point by selecting its layer. In the above case it is the Shape layer.
Clicking on an alignment tool will align all the object to the anchor points selected boundary.
You can use the Background layer as an anchor point to move selected object, but you can't move the Background. An example would be to anchor text to the background and then press Vertical Center and Horizontal Center and the object will be in the center of the frame.