(The following is taken from the Encyclopedia of Christian Apologetics, by Norman L. Geisler)
There are differing kinds of apologetics systems, and no universally-acknowledged way to categorize them. Divergent approaches seem to be determined by the perspective of the one categorizing them. Nonetheless, there are some generally understood terms one can employ to view in a meaningful way the distinctives among more popular approaches.
Apologetics is the discipline that deals with a rational defense of Christian faith. It comes from the Greek word apologia which means to give a reason or defense.
The most important reason to [be involved in] apologetics is that God told us to do so. The classic statement is 1 Peter 3:15, which says, “But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear:”
This verse tells us to be ready. We may never run across someone who asks tough questions about our faith, but we should still be ready to respond if someone does. Being ready is not just a matter of having the right information available, it is also an attitude of readiness and eagerness to share the truth of what we believe. We are to give a reason to those who ask the questions. It is not expected that everyone needs pre-evangelism, but when they do need it, we must be able and willing to give them an answer.