An orquesta (típica) is an ensemble of musicians that plays 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.