/ti voglio bene/ and /ti amo/ may both be used with nearby meanings in
romantic contexts;
-- inside the family, (obviously excluding wife and husband) only /ti voglio bene/ is used; the same rule holds between friends;
--- the literal meaning of /ti voglio bene/ is : /I want you to be {happy, fine, etc.}/
I think it is a marvelous expression (if the feeling is authentic, of course!), as it does not imply the desire of possession that romantic love might imply;
in so-to-say everyday usage they carry exactly the same meanings;
if you would like to explicitly stress the difference, the variation /voglio il tuo bene/
is available.