A .PSD file is an Adobe PhotoShop Document file. It acts as a container to all things Photoshop. Including layers with masks, transparency, text, alpha channels and spot colors, clipping paths etc.
A .PSB file is an Adobe PhotoShop Big file. It is an extension within the Adobe Photoshop product to handle images that are very large. see here for some information about how Photoshop uses its header files to point to the different elements within the container file: Adobe Photoshop File Formats Specification
