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.
A file with .emz extension is a compressed container of Enhanced Metafile (.emf file). It follows .GZIP compression algorithm and takes less storage space due to compression. The small file size makes it more easy to transfer EMZ files over the network.
You can also convert PST into many other file formats. Please see the complete list below.