An approximate translation of 'Ti voglio bene' might be 'I'm very/really/so fond of you' or 'I really care for you'.
Another possible way of distinguishing between 'Ti voglio bene' and 'Ti amo' is using the English distinction between 'I love you' and 'I'm in love with you'.
Ti voglio bene ma non ti amo = I love you but I'm not in love with you
You would say that you love your brother, sister and best friend, in the same way as you might say 'Ti voglio bene' to these people. But if you are 'in love with' someone, it suggests the passion and exclusivity of 'Ti amo'. Cosa ne pensate?