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Wikipedia has an article about ABBA (wikipedia:ABBA). You should read that.

WaterlooEdit

"Waterloo" was ABBA's first big hit. They won Eurovision in 1974 with that song. "Ring Ring" was also popular that year.

Head Over HeelsEdit

"Head Over Heels" was a song they did in the 1980s. I don't think it was as popular as "Waterloo" nor "Ring Ring". The video probably didn't have enough handclaps in it.