(sry it's a stupid question)
A .PSD file is a layered image file used in Adobe PhotoShop. PSD, which stands for Photoshop Document, is the default format that Photoshop uses for saving data. PSD is a proprietary file that allows the user to work with the images’ individual layers even after the file has been saved.
OSD pack means a collection of PSD'S by which you can refer how i made the sig or the photoshop file