I found the definitions on this website:
Pittsburgh Symphony Orchestra and this one:
Enjoy the Music
Concerto: A multi-movement work for solo instrument and orchestra.
Symphony: A multi-movement work (typically four or more movements)for orchestra which has evolved over time.
Classical-era symphonies typically had a four-movement form, which later evolved in the Romantic and
Modern eras to more (or fewer) movements.
Canon: a musical form in which a tune in imitated
by individual parts at regular intervals; known as
a round when each part is continuously repeated.
In simple examples, such as "London Bridge is
Falling Down," the successive voices enter at a
same pitch and at the same speed. In more
elaborate examples, such as the canons in
J.S. Bach's keyboard work known as the Goldberg
Variation, the voices may enter at different pitches
and present the tune at different speeds or even
backwards or upside down (in inversion).
Fugue: A contrapuntal form, beginning with an
exposition in which each voice enters with the
same subject in turn and proceed in imitation.
Unlike a canon, fugues have free passages of
imitation and passages without imitation. They
commonly have from three to six separate voices.
In more complex examples a fugue may have two
or three different themes, contrapuntally combined.
These are known as double and triple fugues.
Fugues were most regularly written in the later
Baroque period, but, regarded as a demonstration
of compositional virtuosity, have also been written
by most composers since then.
Opera: A drama in which the actors sing and are
accompanied by an orchestra. It was invented at the
beginning of the 17th century in Italy as a court
entertainment by composers such as Monteverdi, who
were attempting to revive classical Greek drama. By
the end of the century it became a widespread public
entertainment. In the first half of the next century, in
the works of Handel and Alessandro Scarlatti, it was
characterized by spectacle and vocal virtuosity. In the
reforming spirit of Gluck and the operas of Mozart
that followed, a new simplicity and psychological
penetration entered into opera. In the 19th and 20th
centuries, opera has been written by practically every
major composer, and, in the hands of Wagner.
became the focal point of some of the most advanced
musical thinking of the day. Opera continues to
fascinate composers, despite the complexity,
difficulty, and expense of mounting new works.
Sonata: An instrumental work for a soloist or two
players. In early examples by Domenico Scariatti.
the piece is in one movement, but in general a sonata
is in three or four movements. The first movement
is generally in sonata form, followed by a slow
movement. In a four-movement sonata, the third
movement is usually a scherzo or minuet (although
sometimes this is the second movement, preceding
the slow movement). The sonata ends with a more
extended last movement, usually at a fast tempo.
This is the structure that many sonatas follow,
although the departure of many of Beethoven's
piano sonatas from the model suggests its limitations.
Sonatas have been written more or less constantly
from the late 18th century onward, and are still
being written; there are distinguished examples by
such contemporary composers as Pierre Boulez,
Jean Barraque, and Elliott Carter.
Toccata: A fast keyboard piece, exploiting rapidity
of performance, runs, and repeated notes.
