会谈 means "meeting and talking" 谈判 means “negotiation”

会谈：example：关于朝鲜核问题的六方会谈 谈判：example：中美贸易谈判 explanation：“会谈”is probably for to or more than two seats to solve one issue monolithicly but not to settle them , while "谈判“is actually for two seats to settle one exact issue and settle it for a long period.

"会谈" is kind of a meeting, normal conversation. "谈判" is a negotiation, the participants want to achieve some goals.