When I download BeOS files from the Internet, they have the file extension ".zip" or ".tgz" (or ".tar" or ".gz"). What do I do with these files?

Specifically, files are generally collected together into a "zip archive (or sometimes a "tar" archive, and then compressed into a "gzip" file). The file extensions you will commonly see are ".zip" (for zipped files), or ".tgz" (for tar-ed and compressed files), ".tar" (for tar-only files), or ".gz" (for gzip-compressed files).

The BeOS includes the Expander application, /apps/Expander, for decompressing and unarchiving these files. Simply double-click the downloaded archive, and Expander will launch and (when you click the Expand button) process the file.

Considerably more information about working with compressed and archived files is available in the BeOS User's Guide, Chapter 4 Using Internet Services, in the section Compressing and Archiving Files.