Heresy - rejection of an essential element (dogma or doctrine) of faith - aka for a Catholic an example would be rejection of transubstantiation. Catholics consider Protestants as heretics. Only Christians can be heretics, those who were never part of the church cannot be.
Schism - division from the faith, ex. the historic division between Roman and Eastern churches over the nature of the Holy Spirit. Roman Catholics and Eastern Orthodox consider each other schismatic. Individuals are not schismatic, it take a splitting of the church itself.
Apostasy - completely abandoning and rejecting the Christian faith if one has already accepted it. So one who converts from Catholicism to Islam would be an apostate. Neopagans such as Wiccans who were baptized are apostates.
As I understand it, Catholics who have "fallen away" and are nonpracticing are not yet fully apostates, unless they declare such publicly, as the sacrament of Confession and healing for reunification is possible. However they cannot take Communion. The divorced, without annulment, cannot take Communion.