Many musical elements contribute to texture, including: the way notes in the harmonies are spread out into different octaves and instruments; whether different instrumental sections (strings, bandoneons, piano, bass) are playing simultaneously or alone; whether the sections are playing in unison (the same notes) or in harmony (playing chords).
Changes in texture are used to accomplish specific musical goals. A few examples: to assist in defining and differentiating the phrases; to make the music more bold or dramatic, or subdued or gentle; to increase or decrease the dynamic (volume) level.
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.
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.