c Create a new archive.
t List the contents of an archive.
x Extract the contents of an archive.
f The archive file name is given on the command line (required whenever the tar output is going to a file)
M The archive can span multiple floppies.
v Print verbose output (list file names as they are processed).
u Add files to the archive if they are newer than the copy in the tar file.
z Compress or decompress files automatically.

Example Usage:

tar -xjvf *.tar.bz2

The -u flag gives me an idea for a project archive script.