Classical music is a very subtle and precise art form with so much material that exact nomenclature is essential. In this context, a “song” is for solo voice and piano. Everything else is a “piece”.
Pieces are classified by personnel, the most common being:
- a sonata: solo piano or solo piano plus another instrument, that instrument is titular.
- string trio: violin viola cello
- string quartet: 2 violins viola cello
- string quintet: as above with either an additional viola or cello
- piano trio: piano plus violin and cello
- piano quartet: piano plus violin viola cello
- piano quintet: piano plus string quartet
- woodwind quintets: flute oboe clarinet bassoon horn
- brass quintets: usually 2 trumpets, horn, trombone, tuba
- sextettes, septets and nonets: various combinations make up a fairly vast but underperformed repertoire
These all fall under the rubric of “chamber music“, which has its own specialists and is generally performed in more intimate venues.
I was initially put off by how dry titles of the pieces were. “Brahms Clarinet Quintet in B-minor Op 115”, but I soon learned how efficiently it lists the composer, personnel, primary tonality and op number, which refers to order of publication and usually, but not always, chronology of composition. And it can give you an indication as to what kind of mood the piece will have. A piece in C Major, for example, invariably has a different vibe than D Minor.
Opera is the plural of opus, fyi.