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.
Files with ICO extension are image file types used as icon for representation of an application on Microsoft Windows. These come in different size, colour support and resolution to suit the requirements of the display. Another similar image file format on Microsoft Windows is .CUR for cursor representation and defines a hotspot in the image header. On MacOS, ICNS file formats serve the same purpose as ICO files. Several online websites as well as applications provide the feature of creating such files and convert other image formats such as BMP, PNG, etc. to icon file format. The official IANA registered Internet media type for ICO files is image/vnd.microsoft.icon.
You can also convert PST into many other file formats. Please see the complete list below.