Devil's advocate is when someone is advocating a side that most people wouldn't.  She may or may not agree with it, but is just trying to show people there isn't a right or wrong answer.  

A party pooper is someone who brings down the mood of a party.  Maybe they won't dance or laugh.  

The main difference is that a devil's advocate is usually participating, but isn't going along with the general crowd.  You can think debate with devil's advocate.

A party pooper is someone who is typically trying to ruin the fun whether on purpose or just having a bad day.

very grateful for your comment.

She is always a devil's advocate in a group because she is likely to give an abrupt response to questions to arouse other's attention.

can you explain the meaning of "advocate" and "pooper" or the idiom "party-pooper" ?