El Choclo was written by Ángel Villoldo in 1898 and was premiered by José Luis Roncallo in 1903.
El Choclo has been a standard for a hundred years, popular to this day and still frequently performed and recorded, often in elaborate concert-oriented arrangements. Tango.info has a partial list, here.
El Choclo is an example of an early tango, using the original as a foundational rhythm, almost constantly. Tango music soon replaced the habanera with a more syncopated variation. The habanera rhythm is seldom heard in any of the more mature El Choclo recordings.
The piano music has been synchronized with several recordings. Follow along with Juan Maglio “Pacho”‘s mid 1910s recording, Roberto Firpo’s 1940 recording, Juan D’Arienzo’s 1954 recording, or Carlos Di Sarli’s 1954 recording.
Milonga came before tango and its characteristic rhythm was, and is, the Cuban habanera. Early tango's rhythm was based on the habanera also. Soon other syncopated patterns dominated. The 1st variation, the "syncopa" is the most commonly heard syncopated rhythm in tango music. All three versions are used in milonga.
Habanera Variations describes the syncopation and relationship between these patterns.
Composers/arrangers make a very conscious decision regarding form. The order sections are heard greatly effects our sensations and responses as listeners and dancers.
Sections are the top level element of music's form. They are the the large building blocks of tango music, typically lasting around thirty seconds or so. Each section is a unique segment of music, having a distinct musical character.
Tango music has two, occasionally three, primary sections, which we may label “A”, “B”, “C”. Sometimes there is an "Introduction", "Bridge", a short section between two larger ones, or "Coda", a short concluding section.
Usually each section will be played consecutively in order (A then B then C), followed by various other orderings. Typically in tango songs each section is played instrumentally then each is sung, then section A is played instrumentally: A-B-A (vocal)-B (vocal)-A. But there are many exceptions and other possibilities.
Phrases exist within a section.
A bar or measure is a small segment of music containing all the number of beats as specified by the time signature.
An orquesta típica is an ensemble of musicians who play tango music. Typically, there is a string section, a bandoeon section, a piano, and sometimes a singer or two. There is no specific rhythm section – no drums or other percussion instruments. An orquesta típica is an expanded version of a sexteto tipico, which includes 2 bandoneons, 2 violins, double bass, and piano.
I call any band that plays tango, no matter what the instrumentation, an orquesta. Not entirely accurate but it simplifies things.