They're not really "keywords". "-f" is the command line option which specifies the output format. "nasm -hf" for a complete list. "-f elf", equivalent to "-f elf32" is one of the supported output formats - "executable and linkable format" - used by Linux and BSD (and perhaps "etc."). ".o" is the usual extension of such a linkable file.
Or perhaps you're seeing them in some other context?
Best,
Frank