What is a Personal Folders File (PST)?

Do you have UFOs on your disk? To make more sense of these unidentified file objects, you can use a smart tool called DROID.
DROID looks at a file's contents and, if available, its extension (e.g. "txt") to tell the format. The format, in turn can tell you about the file's origin, and how you might be able to open it.
Speaking of file formats and extensions: what are these .pst files in my account? "Microsoft Outlook Personal Folders," says DROID:
›› Outlook uses PST or Personal Folders files to store all data from email to contacts and calendars.