tar by the name is "tape archive" so it assumes, if "-f" option wasn't provided it tries to put file on the tape drive: /dev/rmt is a acronym of tape drive in UNIX.

So you give it "-f" option and a file name to redirect archive to file and not to tape.

Today's tar is quite improved so there's no need to use compression as a separate program. Just add "z", like "tar -cvzf". Some versions of tars take the list of parameters without "-" but normally it is required. Get "UNIX Essentials and UNIX Core" DVD or "UNIX in 24 hours" as they teach all this stuff.

Hope it helps.