Using layers

Hi, when adjusting a layer, what´s the difference between using "Image>Adjust " and "Layer>New Adjustment Layer"?

Adjustment layer is a layer that has no content, but only works as a layer of that effect..you know? lol.
Adjusting a layer means you adjust the contents of that layer.

Adjustment layers are great, because you can always delete them if you want without losing any content.

..damn I made that sound complicated... lol