Who decides which is to be called what?
Symphony: Full orchestra piece not centred on one instrument.
Concerto: Solo instrument accompanied by an orchestra.
Sonata: Solo instrument usually accompanied by piano (or another keyboard instrument). Can also be a solo keyboard piece.
As others have said,
A symphony is a piece written for an orchestra.
A concerto is a piece written for an orchestra and one select solo instrument.
But as others have not yet said,
Sonata has a few meanings, actually, that are highly cultural and time-period specific:
- Prior to the classical era it could refer to a complete piece of any non-vocal music (vocal music was called a 'cantata' by contrast).
- It could mean a piece written for a piano (think beethoven's moonlight sonata) and maaaaaybe a solo instrument. Rarely.
- It could refer to a specific form, also known as Sonata-Allegro (https://en.wikipedia.org/wiki/Sonata\_form). Note that sonata FORM was very popular in the classical period for sonata PIECES, but also was used for symphonies, concertos, quartets and quintets, etc.
- Finally it could also refer to an entire multi-movement piece in which the first movement is (usually) sonata-allegro, which kind-of brings us back to the first definition.
Music history is fun!
To answer your last question, the composer decides. Always. The composer may be wrong, though I can't think of any examples in which a song has been 'mislabeled'
A lot of the answers are inaccurate, particularly in defining a “symphony” far too loosely. Copying from my answer here:
The words “sonata”, “concerto”, and “symphony” refer to specific musical forms with their own typical scale and instrumentation. In the classical era, these terms tend to be pretty precise and specific, but in the later Romantic period (and certainly in the Modern era), composers took much greater liberty with the traditional forms. I'll explain each one of these forms, the general rules, and some types of exceptions.
A symphony is a major large-scale work for a full orchestra, traditionally in four movements. Composers who wrote symphonies typically considered them to be their greatest and most serious works.
Symphonies are numbered, not usually named, though sometimes the composer will add a subtitle (e.g. “Heroic” for Beethoven's third). Symphonies are almost always pure music with no narrative. For a counterexample, see Berlioz's “Symphony Fantastique”, which has no number and is meant to tell a story.
Symphonies are almost always composed of several distinct parts, or “movements”:
Sometimes the second and third movement are swapped. Sometimes an extra movement is added before the finale. Sometimes two or more of the movements run together with no gap. Early Classical symphonies tend to be pretty strict, but later Romantic symphonies take a lot more liberties.
Not all large-scale works for orchestra are symphonies. For instance, Rimsky-Korsakov's Scheherazade looks an awful lot like a symphony, but it's not a symphony because R-K didn't call it one.
Symphonies may use extra instruments beyond the “standard” composition of an orchestra, but rarely vocalists. There are several well-known exceptions, including Beethovan's ninth and several of Mahler's.
A concerto is a work for a solo instrument and a large ensemble (such as an orchestra). Traditionally, a concerto has three movements — like a symphony, only without the scherzo.
Like symphonies, concertos are typically numbered and not named. (Concertos for different instruments are numbered separately.)
Sometimes there may be a concerto with two or more solo instruments, such as Mozart's concerto for flute and harp (plus orchestra).
In the baroque era, “concerto” sometimes meant simply a work for an ensemble without a particular solo instrument. “Concertino” usually means this as well.
A sonata is a work for a solo instrument, usually with piano accompaniment (unless the solo instrument is a piano in the first place). It will usually have several movements. Three-movement sonatas that resemble concertos are common. Baroque sonatas often have many short movements instead of a few longer ones.
In addition, sonata form is a structure that a piece of music (like one movement of a symphony) might have. Music in sonata form has two main themes and is structured like this:
The first movement of a symphony or concerto is quite often in sonata form, and it's frequently seen elsewhere as well.
If you have more questions or want clarification on any of the above, I'd be happy to provide.
Sonata is a musical form, meaning that the way the music is arranged is in a certain order. (Exposition, Development, Recapitulation) It's a way for a composer to fully explore musical themes and show off their compositional skills. Sonata can also refer to a group of pieces (usually 3 movements), the 1st movement being in Sonata form, the 2nd movement usually being slower shorter piece, and the 3rd movement being in some sort of Rondo form. Throughout these 3 movements there can be re-accuring themes and harmonic progressions linking all 3 movements together.
Symphony is in reference to the full orchestra, not to be confused with symphonia which is also a musical form.
Concerto is a style of composition that involves a symphony, but emphasizes one or two specific instruments which are the "soloists"
And what about the differences between Minuette, Bolero, Requiem, Nocturne, Serenade, etc?