here what psds means
a PSD is a Photoshop design file.
and i think it's lots of layers .. contains textures , effects and adjustments and fonts sometimes ...
when u play with adjustments and texture and come out with good coloring and u want to use same stuff on other pic u can simply save them as a group with PSD format .. and PSD format can be edited as well means when u open it again u can change all ur effects and adjustments and everything .. it's something magical about PS and u also can desgin web with it
hope i explain it well :D