I have difficulities understanding the difference of meaning between these two expressions.
Are there any? If no difference of meaning, is it a difference of usage?
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"Was geht ab" is a colloquial question usually asked by someone arriving, addressed to the people already present, meaning something like "hey guys, what are you currently doing/planning to do?".
"Was ist los?" means "what is the matter?", asked when you meet someone an see that person's concerned or troubled face or you sense in other ways that something is bothering him/her.
A slight variation of that would be said if you arrive and realize that something strange is currently happening: "Was ist denn hier los?", meaning "What is going on here?".