To greet someone in Korean, say "Ahn-nyung-ha-say-yo?" It's always spelt with a question mark because you're actually saying, "Are you peaceful?" It's a much sweeter greeting than our boring "Hello!" (But, answering a phone is a little bit different. In this case, you say "Yuh-bo-say-yo?" which translates to "Hello?")
As for goodbyes, it depends on whether you're staying and saying goodbye to someone who's leaving or the other way around. If you're staying and the other person is leaving, say "Ahn-nyung-hee-gah-say-yo." If you're the one who is leaving, say "Ahn-nyung-hee-kay-say-yo." Ahn-nyung (peace) is found in these expressions, too. Instead of a simple "goodbye," you're saying "Peace go with you" or "Peace stay with you," respectively.
It's no wonder that flashing the peace sign is such a popular pose in photos!