A woman may be divorced only by receiving a bill [of divorce]. This bill is called a get.
The Torah establishes ten principles as fundamental [for a divorce to be effective]. They are:
a) That a man must voluntarily initiate the divorce;
b) That he must effect the divorce by means of a written document and through no other means;
c) That this document must communicate that he is divorcing [his wife] and releasing her from his domain;
d) That it should utterly sever the connection between the husband and his wife;
e) That [the get] should be written for the sake [of the woman being divorced];
f) That once [the get] is written, there should be no action [necessary] except its transfer to the woman;
g) That he should actually transfer [the get] to her;
h) That he should transfer [the get] to her in the presence of witnesses;
i) That he should actually transfer it to her for the sake of divorce;
j) That the husband or his agent should be the one who gives it to her.
The other requirements of a get - e.g., dating it, having it signed by witnesses and the like - are all Rabbinic institutions.