Wikipedia has an article about ABBA (wikipedia:ABBA). You should read that.
Waterloo[edit | edit source]
"Waterloo" was ABBA's first big hit. They won Eurovision in 1974 with that song. "Ring Ring" was also popular that year.
Head Over Heels[edit | edit source]
"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.