On Genre

It gets complicated because composers are a creative bunch, and don't always follow a set format, but I and others usually recognize some particular genre in music:

Chamber
Orchestral
Song
Opera
Sacred

These are the basics, but they can be broken down.


Chamber: 


Chamber music refers to small instrumental ensembles or a single instrument.


The following are common instrumental breakdowns for chamber ensembles:

Solo (one instrument - obviously not an ensemble).
Duo of various instruments.
Trio (3 instruments)
String Quartet or quartets featuring other instruments.
Quintet,  Septet, Octet - ensembles of either strings, winds,  or a combination of the two and some with piano.
Chamber Orchestra.

Chamber pieces can be further broken down into these and other genre:

Sonatas of one or more instruments - We'll discuss the sonata form in a later thread.
Partitas
Preludes
Etudes
Suites
Variations on a theme
Waltzes
Nocturnes (Notturni)
Romances
Rondos (Rondeau)
Scherzos (Scherzi)
Divertimentos (Divertimenti)
Cassations


Orchestral (Symphonic):


Orchestral or Symphonic music involves larger ensembles of either full Symphony Orchestras or String Orchestras.  The typical genre are as follows:

Symphonies
Overtures - Often the opening movement of an opera or other stage work, or written as a single piece.
Concertos (pl. Concerti is also correct) - One or more solo instruments with full orchestra or strings.
Serenades
Divertimentos (pl. Divertimenti is also correct)
Cassations
Marches
Incidental music for Ballet or other Stage works.
Suites
Individual orchestral movements either in or not in sonata form.
There can also be orchestral Rondos, Scherzos (Scherzi), Waltzes, Nocturnes (Notturni), Variations, etc.


Song:

Many classical composers have written individual songs or song cycles.

Most often they are performed by a vocal soloist (either Soprano, Alto, Contralto, Tenor, Barritone or Bass) accompanied by a piano.  However, they can also be accompanied by other instruments or an orchestra.  They can also include more than one vocalist,  as in a duet, trio, quartet or larger vocal ensemble.


Opera:

Opera gets even more complicated, but the basic types of opera are:

Opera Buffa
Singspiel

The difference is that in Buffa, all the vocal parts are sung, while Singspiel includes spoken parts - similar to the modern musical.  In Buffa, the recitative replaces the spoken part.  Recitatives are often sung somewhat improvised with an accompaniment of chords on a harpsichord or other instrument, or with no accompaniment.

Operas are stage works depicting dramatic, and sometimes comical events - usually fictional, and are based on either an epic poem, story,  or a written work called a libretto.  A libretto is similar to a script or screenplay, so it's written specifically for the opera.  Librettos are often written by an author other than the opera composer.

Arias are the main vocal solo pieces of an opera.  Quite often arias are written to be performed as separate pieces from operas, and are usually called Concert Arias.


Sacred:

The basic sacred piece is the Choral Mass.

Masses can be performed a capella (without accompaniment) or with organ, piano or orchestral accompaniment.

The mass is often broken down into 6 separate sections in the following order:

Kyrie
Gloria
Credo
Sanctus
Benedictus
Agnus Dei

However, many masses diverge from this format.  Masses can also feature arias sung by a vocal soloist.

Any other piece of vocal music, which covers sacred themes could also be considered sacred music.

A few examples are:

Sacred Oratorios
Sacred Vespers
Individual sacred songs
Motets
Benedictions

There are also instrumental sacred pieces, such as Mozart's Church Sonatas.