晚上好: You started a conversation at night with your friend. 晚安: =good night When your friend is going to sleep right away, you can use it. Then you ended the conversation.

晚上好 = saying hello in the evening, night etc. Don't try to translate directly from English's "Goodnight". 晚安 = goodnight. Literally means "to sleep well in peace" or "may peace be in your sleep" This is cultural biase. For example "吃了沒有?" to mean "hello" and not to mean literally. The listener may find it puzzling if they had not experience anything bad happened to them to warrant such words.

Just to add to Mao and many buddies's answer. 晚上好 is equivalent to the night version "你好“ [ saying hello ] 大家晚上好 is equivalent to the night version of "大家好” [ greeting to a group of audients. For example, on a TV show ] don't confuse it with 早安"Good morning" and 午安"Good Afternoon". as a summary: ================ 早上好 = 早安 = 你好 = good morning [ saying hello in the morning ] 午安 = 你好 = good afternoon [ saying hello in the afternoon ] { There is no "下午好"} 晚上好 = 你好 ～= good evening 晚安 ～= good night and sometime imply "goodbye"

Just to add to MAO's answer. 晚安 can be used when you're leaving a conversion. Your "friend" doesn't really have to go to sleep right away. lol