Orchestration or instrumentation is how the instruments are used; the instruments playing at any given time and their function, such as melodic, accompaniment, creating the pulse, linking phrases (fills). The core instruments used in tango – bandoneons, strings, piano, sometimes voice – and how they are used is what creates the typical tango sound. The s also have unique ways of orchestrating the music, which is part of their individual style.
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.