Classical Music

Classical music is the term generally used these days to refer to a particular tradition of music from Western European civilizations, and is often contrasted with the equally vague term Pop music. In the current era, much of this body of music has been studied carefully in scholarly manner, and is performed and appreciated as standalone art, even if it wasn't originally conceived as such.

The dates which characterize "classical music" are arguable; while some maintain that it began when norms about concepts such as rhythm and tonality began to be established, around 1600, others extend it much farther back to include medieval art music. Some modern composers also believe that there is no longer any value in considering contemporary art music "classical."

In contrast to current popular styles, classical music typically shares some traits and has some differences, such as:
 * "common-practice" tonality, i.e. all the major and minor keys that we know and love. These were developed during this historical period.
 * Classical pieces tend to be longer. Also, far more of them are instrumental, so they are far less likely to be called "songs". Also, many of them are named after their form, rather than a unique title, so you'll get lots of things like "Sonata #4 in E-flat major". (That's just the way people rolled back then.)
 * Classical pieces are less likely than pop/techno/rock/metal songs to use percussion to establish rhythm. There's rhythm alright, but this is established through the use of pitched instruments.
 * Classical pieces may often also be more complex musically, especially in terms of pitch and form. (Timbre is probably more complex in pop/techno/rock and non-European music; rhythm is kind of a wash.)
 * You probably already know this, but the stereotype of classical music includes such instruments as bowed strings (violin, viola, cello, (bowed) bass, etc.), and woodwinds (flute, clarinet, bassoon, etc.), and such ensembles as the string quartet (violin x2, viola, cello) and the symphony orchestra (bowed strings of all kinds, woodwinds of all kinds, some brass, and a few kettledrums).
 * The size and composition of the orchestra is also a good means of identifying which period a piece comes from, although not a perfect one.
 * Medieval/Renaissance: Voices only, or folk-ish instrumentation added by modern performers (either based on nebulous evidence like illustrations and song texts, or simply because it sounds good)
 * Baroque: Flutes, oboes, strings, horns, valveless trumpets, harpsichord and timpani.
 * Classical: Addition of clarinets and bassoons, replacement of harpsichords with pianos.
 * Romantic: The fullest orchestra, which added trombones, valved trumpets and horns, tubas and lots of extra percussion. Basically it had just about everything.
 * Modernist: Harder to pigeonhole - but anything that uses electronic instruments has to fit here.

Classical music is not a single style, but is in fact a bunch of different styles, generally classified into several periods:
 * Medieval c. 900 AD up until the end of the 14th century: mostly monophonic vocal music, and most of that religious. Choral music starts showing up towards the end of this. By modern standards, it's all pretty severely weird: common-practice tonality shows up after the Medieval period. Perotin, Vitry, and Machaut are all good examples of the choral stuff. The monophonic music is mostly Gregorian chant, or at least heavily influenced by it.
 * the Renaissance (1400s-1600s): Characterized by choral music in styles that we might find hard to relate to today (although most find it more approachable than the medieval stuff). Again, a lot of the music was composed for churches. Josquin des Prez is your go-to composer here.
 * the Baroque Era (mid-1600s to about 1750): Still a lot of church music, but the wealthy nobles and royalty often found time and opportunity to indulge in fancy music for the heck of it. For a stereotypical Baroque sound, look for anything by J.S. Bach or his contemporaries. Famous works include the Toccata in D minor by Bach, the Canon in D by Pachelbel, and the Messiah oratorio by Handel (which includes the famous Hallelujah Chorus). The Concerto (a piece where a solo instrument or small group of instruments alternates passages with a larger orchestra) peaked during this period. A common feature was counterpoint, terraced dynamics (the music is either loud or soft, with few crescendos or diminuendos or so forth) or having multiple melodies running at the same time (producing a texture called polyphony). During the early Baroque, Claudio Monteverdi began to write the first notable operas. The harpsichord is a stereotypically Baroque instrument. If you hear a piece of classical music and there's a harpsichord in it, it is almost certainly a Baroque piece.
 * the Classical Era (or Classical Period, not to be confused with Classical Mythology) (about 1750 to the early 1800s): Think of Wolfgang Amadeus Mozart and Ludwig Van Beethoven. While counterpoint was still respected, this era saw the growth of homophony, or having a melody on top of a chord-based accompaniment. Joseph Haydn was very influential in the development of the sonata-allegro form, which became a dominant musical form in this period. The piano replaced the harpsichord as the dominant keyboard instrument, and music became increasingly independent of religious activity. Concertos began giving way to Symphonies. Music also began to be written for ensembles such as the traditional string quartet.
 * the Romantic Era (or Romantic Period) (the 1800s): Composers started pushing the limits of their styles and instrumets. Sounds became lusher, textures denser, harmonies became more chromatic and orchestras bigger, and the sustaining pedal on the piano (the one that holds notes down without having to keep your fingers on the keys) became popular. Famous composers included Fryderyk Chopin (Fantasie Impromptu, Revolutionary Etude), Franz Liszt (Dante Sonata), Johannes Brahms (Academic Festival Overture), Richard Wagner ("Ride of the Valkyries" is from one of his operas, and his style set the standards for epic film music through today), Pyotr Ilyich Tchaikovsky (The Nutcracker Ballet), among others. Programmatic Music (music telling a story or depicting scenes, as opposed to abstract music) and music reminiscent of particular folkstyles became popular. Some people consider Beethoven to be the first Romantic Era composer, because he started or inspired many of the trends to come in this period. It's also important to note that the Romantic style never really goes away. Sergei Rachmaninoff and Edward Elgar were essentially romantics, yet wrote most of their music in the 20th century. Andrew Lloyd Webber and John Williams are romantic to their very bones.
 * the early twentieth century (exactly when it says on the tin): Composers pushed the limits of musical understanding and style even more, resulting in things like atonality (having no key at all) and neoclassicism (imitating, though with obvious differences, earlier styles). Claude Debussy and Maurice Ravel, the most famous impressionist composers, began to discard the common-practice tonality of earlier periods. Serialism (methods of composition based on the continuous transformations of series of musical elements) began to be used heavily, starting with the twelve-tone system of Arnold Schoenberg and his Second Viennese School. Trends were all over the place; some composers became very interested in Folk Music and Ethnic Music (such as Bela Bartok), others channeled the social and political turmoil into their works (Sergei Prokofiev and Igor Stravinsky), and some combined these with popular styles (Scott Joplin and George Gershwin—whose origins are with Ragtime and Jazz respectively, had heavily classicizing elements in their work). That last category would eventually give rise to the Show Tunes of Broadway. Picking a representative example our of all this variety is impossible: the defining characteristic of early 20th century classical music is the absence of defining characteristics. But try out the ballets of Stravinsky (The Firebird, The Rite of Spring, Petrushka) for just one style.
 * The later twentieth century to the present, during which all the tendencies of the earlier part of the twentieth century were pushed even farther. During the 1960s, the mainstream classical music world was heavily influenced by the Darmstadt School, whose members, such as Karlheinz Stockhausen and Pierre Boulez, pushed the serialism of Arnold Schoenberg and his pupil Anton Webern to its extreme. Edgard Varese, a major influence on Frank Zappa, began to experiment with electronic music, while other composers began to use scales different from the traditional 12-note equal temperament scale. Iannis Xenakis pioneered the use of mathematical modeling in music. Meanwhile, minimalist composers such as Steve Reich and Philip Glass experimented with music that used very few basic elements, often repeated with variations. In the 70s and 80s, composers such as Alfred Schnittke and Luciano Berio began to write polystylistic pieces that drew on many prior musical traditions in a Post Modern way. The boundaries between popular and classical music began to blur; for example, John Cale, violist of The Velvet Underground, was associated with the minimalist La Monte Young, and The Beatles were influenced by Stockhausen. Lately, the classical music world has seen a resurgence in the popularity of music combining a romantic feel with modern techniques, written by such composers as John Corigliano. Other notable composers include John Cage, Morton Feldman, and Giacinto Scelsi. As in the early twentieth century, there isn't any truly representative music, but the Sinfonia of Luciano Berio might be a good place to start.

Unless you're making a conscious effort, ninety percent of the classical music that you actually hear will come from the Baroque, Classical, and Romantic periods. The Early Music (Medieval/Renaissance) and Modernist scenes are pretty disconnected from mainstream classical music, and have their own specialized performing ensembles and record labels.