The initial '-' is optional in tar.
So 'tar xvf file.tar' is the same as 'tar -xvf file.tar'.

'x' -> extract
'v' -> verbose (i.e. list the files as they are extracted. Without it it's silent.)
'f''<name> -> name of file to extract from (unpack)
't' -> list content (tv -> list verbose)
'z' -> the tar file is compressed with gzip (e.g. .tar.gz, .tgz)
'j' -> the tar file is compressed with bzip2 (e.g. .tar.bz2)
'c' -> create (make) a tar archive: tar cvf newtarfile.tar <list of files to archive, or directory name etc.>
(Add z or j if you want it compressed as you go along.)

Note that there are some pecularities in tar's option handling: If you specify two or more parameters that take an argument, e.g. 'b' (block size) and 'f' (file/device name), then the arguments are listed together at the end: tar tbf 20 file.tar

The busybox version of 'tar' on the N800/N810/770 is rather limited, it doesn't have that 'b' option for example. More important, it doesn't have built-in help. If you install GNU tar then just enter 'tar --help' to see the options.