Columns in pg_shadow?

I have been looking for information on the meaning of each column in
pg_shadow, but have been unable to find any thus far.  It would be good if
someone could enlighten me, or point me to existing documentation.  The
columns I am unsure of are listed below:

usecreatedb:
    Ability to create new databases?

usetrace:
    ?

usesuper:
    ?
     I assume this makes the user a "superuser," but I am not sure exactly
what that means.

usecatupd:
    Ability to change system tables?


       Thanks,
                  -Mike