Good question! (You picked a nice difficult passage :) From my studies : There is more than one Greek word that is translated "preach". One (transliterated euangelizo) is basically to preach the good news concerning the Son of God. The one used in First Peter (transliterated kerusso) is also translated preach (with the synonyms being herald, proclaim, publish). From what I can find out, this is Jesus preaching (proclaiming) His victory. As one person had it, Jesus probably published "a message of judgment and final condemnation in light of his finished work on the cross to these disobedient spirits." Now as to who these spirits were there is quite a bit of discussion. Most feel that it was the people in the time before the flood that the Spirit (through Noah) had preached to. The "prison" being hades - the abode of the dead. However, there is mainly speculation, not concrete answers. For the full story, just be sure live in God, and ask Him in heaven!
good and difficult question... even as one of Jehovah's witnesses I had to do some research!
the spirits referred to the demons. in the next scripture it talks about those whomever disobedient in the days of Noah (the sinned angels an their offspring, the Nephilim). this scripture means that when Jesus died, when he was in spirit form, he preached to the demons their coming abyssal and later destruction, their judgement statement.
it does not refer to humans or ghosts... because the only humans to enter the spirit realm were the 144,000 who became kings and priests in heaven (revelation 14)
the context of this passage is encouragement, the preceding scripture exaults Christians to endure because "even Christ had to die once" and supports it by describing his condemning the demons