Although Boa concedes that not everyone agrees that apologetics involves all four of these functions, I found them helpful in clarifying what one can aim to accomplish using apologetics.
"The first function of apologetics may be called vindication or proof, and involves marshaling philosophical arguments as well as scientific and historical evidences for the Christian faith. The goal of this function is to develop a positive case for Christianity as a belief system that should be accepted. Philosophically, this means drawing out the logical implications of the Christian worldview so that they can be clearly seen and contrasted with alternate worldviews.
The second function is defense. This function is closest to the NT and early Christian use of the word apologia, defending Christianity against the plethora of attacks made against it in every generation by critics of varying belief systems. This function involves clarifying the Christian position in light of misunderstandings and misrepresentations; answering objections, criticisms, or questions from non-Christians; and in general clearing away intellectual difficulties that nonbelievers claim stand in the way of their coming to faith.
The third function is refutation of opposing beliefs. This function focuses on answering the arguments non-Christians give in support of their own beliefs. Most apologists agree that refutation cannot stand alone, since proving a non-Christian religion or philosophy to be false does not prove that Christianity is true. Nevertheless, it is an essential function of apologetics.
The fourth function is persuasion. By this we do not mean merely convincing people that Christianity is true, but persuading them to apply its truth to their life. This function focuses on bringing non-Christians to the point of commitment. The apologist's intent is not merely to win an intellectual argument, but to persuade people to commit their lives and eternal futures into the trust of the Son of God who died from them." 
Courage and Godspeed,
1. Kenneth D. Boa, What is Apologetics?, The Apologetics Study Bible, p. xxv.