Formal:
* Maayong gabii (pronounced mah-yo-ong ga-bee) - This is the most common and respectful way to say "good evening".
Informal:
* Maayong hapon (pronounced mah-yo-ong ha-pon) - This literally means "good afternoon", but can be used as a greeting in the evening if you're speaking to someone you know well.
* Gabii (pronounced ga-bee) - This is a simple and casual greeting, similar to saying "evening".
It's always a good idea to err on the side of formality, especially if you're not sure of the person's age or social standing.