Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. User information is stored in folders of different types that include emails, calendar items, notes, contacts, and several other file formats. PST files are used for archiving emailing data offline that can be later loaded and viewed in various applications.
PSD, Photoshop Document, represents Adobe Photoshop's native file format used for graphics designing and development. PSD files may include image layers, adjustment layers, layer masks, annotations, file information, keywords and other Photoshop-specific elements. Photoshop files have default extension as .PSD and has a maximum height and width of 30,000 pixels, and a length limit of two gigabytes.
You can also convert PST into many other file formats. Please see the complete list below.