offsetof

hi all,
i have another question.
I've read the FAQ regarding my former question (sizeof struct and
union) and in question 2.14 it talks about offset macro.

#define offsetof(type, mem) ((size_t) \
((char *)&((type *)0)->mem - (char *)(type *)0))

Can anyone explain me how it works.
1)I can't understand what "0" casted to (type *) means
2)Isn't the second menber of substraction "(char *)(type *)0" useless?

.