Apostasy (irtidad) is the act of turning away from Islam having accepted it. This involves the rejection of tawhid and nubuwwa and the essentials of Islam (e.g. that the Qur'an is a book of God). There are 2 types of murtadd - fitri and milli. A fitri murtad is one who was born of Muslim parents (or at least one parent) and then rejects Islam. If he rejects Islam, then, according to the fatawa of our maraji (including Ayatullahs al-Khu'i and Khumayni) he is to be killed.
Murtad is the person who rejects his belief or part of his belief.