They both are file formats to store digital images. There is very less difference between PSB and PSD, what they differ is how they are stored and used. PSD is a default extension for Photoshop files and it can support the file size of 30,000 by 30,000 pixels. While for creating big documents or large image .psb file format is used, and it can save an image size upto 300,000 by 300,000 pixels.