La Cumparsita is a very recognizable tango song. It was written in Uruguay by Gerardo Matos Rodriguez in 1917. He was approximately seventeen when he composed it, so he wouldn't have realised how famous his song would get. 'La Cumparsita' means 'The Little Parade'. Gerardo first played his tune in a coffee shop called Confitería La Giralda in Montevideo, Uruguay. Some seven years later, Gerardo found himself in Paris and found out that his song was a massive hit. He also found out that Enrique Maroni and Pascual Contursi, two lyricists, had added different lyrics and renamed the tune 'Si Supieras'. Gerardo Rodriguez spent a long time afterwards in courts, trying to gain the original author's rights to the music. He eventually did, but there are still debates on whether his version or Enrique Maroni and Pascual Contursi's version were the actual hits.
La Cumparsita is considered to be the anthem of Tango, and it is almost always used as the tango music in tango dance scenes in movies. There have been many different versions, some simplified, some with different instruments, and I find that two different versions can sound completely different.
Here is what it should sound like in general played on the piano: