Classical

This is music composed c.1750-1810, with the main composers being Mozart, Haydn and Beethoven (Beethoven lived until 1827 and also wrote music in what we would call a Romantic style). Some of the characteristics of Classical music include grace, symmetry, beauty and a certain simplicity (certainly compared to Baroque or Romantic music).

Here's a playlist of music by Mozart and Haydn to give you a general overview of the style. More detailed playlists follow this one (about Sonata Form, Minuet & Trio etc.)

Sonata Form

The SYMPHONY developed during the CLASSICAL era, and has been a common form of orchestral music ever since. During the CLASSICAL era, it was usually a four movement structure, with the first movement written in SONATA FORM, as in this example. In this video, we hear the EXPOSITION and part of the development section.

The first SUBJECT is heard at the start of the clip, followed by a BRIDGE/TRANSITION passage (starting at 0.24) that modulates to the RELATIVE MAJOR for the second SUBJECT (starting 0.50 - notice the use of DESCENDING CHROMATIC figures here).

The development section starts at 1.59: in this case, the first SUBJECT  is used sequentially to pass through a number of different keys before it is treated CONTRAPUNTALLY (2.11), and then the DESCENDING SEMITONE with which it starts is used to again MODULATE through different keys (2.33).

SONATA FORM was also the normal structure of the first movements of SONATAS during this era. In the example below, the whole of the EXPOSITION is repeated before the development section is heard (the repeat starts at 2.12). 

The different parts of the form are heard as follows: first SUBJECT at the start; BRIDGE/TRANSITION passage, MODULATING to the dominant at 0.47; Second SUBJECT at 0.53; development section starts at 4.06; recapitulation starts at 5.04, with the second SUBJECT - this time in the tonic key - at 5.49.

A couple of other things to notice: the BROKEN DIMINISHED CHORD at 4.27-4.30; the dominant PEDAL, starting at 5.04, which leads into the recapitulation; the short CODA, based on a tonic PEDAL (starting at 6.57) that ends the piece.

MINUET & TRIO

SONATA FORM (see previous post) was frequently used in CHAMBER MUSIC as well, particularly for first movements. In the video below, though, we have a third movement, and as in many SYMPHONY and SONATA third movements,  the structure is MINUET & TRIO.

The MINUET, a graceful dance-like section, with three beats in a bar, is followed by another, the TRIO, which is often in a related key (eg. RELATIVE MAJOR/MINOR, DOMINANT); the MINUET is then played again, creating a TERNARY FORM movement.

In this example, we hear a partial repeat of the MINUET (starting at 1.01) - notice the use of a PAUSE here (1.07) and earlier at 0.21. Rather unusually, the TRIO (1.50) is in Eb Major, which is neither the RELATIVE MINOR nor the dominant key of G major. The return of the MINUET starts at 3.00, with the partial repeat missed out this time.

Below is another MINUET and TRIO by Haydn, this time the 3rd movement of a SYMPHONY. The TRIO begins at 1.53 and remains in the home key of Bb Major. The MINUET returns at 3.24.

SCHERZO

Beethoven and Schubert were key figures in the development of late CLASSICAL style and early ROMANTIC style. One of their innovations in SYMPHONIES was to replace the third movement MINUET & TRIO with a similar triple-time movement known as the SCHERZO. SCHERZOS are also heard in CHAMBER music, as in this first example, and SONATAS. 
Like the MINUET & TRIO they are in triple time (three beats in a bar) and tend to be even more lively. In Italian, Scherzo means 'joke', so they're normally quite lighthearted.




Here's a playlist with further examples of SCHERZOs by Beethoven and Schubert.