Opera, as an art form didn't get its start until around 1600 when the Florentine Camerata decided to recreate Ancient Greek drama in a musical form. A collection of Italian aristocrats including such families as the Sforzas, the Viscontis, the Gonzagas, the d'Estes and the de Medicis decided to abandon the earlier complexities of polyphony and counterpoint in favor of new harmonic structures. They lead to one of the most important achievements of the Baroque period: the opera.

The advent of the continuo, combined with the abandonment of the interweaving of voices in counterpoint form (made popular during the Renaissance) provided a framework for musical improvisation that hadn't been seen in many years. Likewise, without complexities of polyphony to deal with, major and minor keys came to the forefront, as the each chord assumed function relative to the tonic (first) and dominant (fifth) notes.

Recitative
Resitatives are the part of the opera used to advance the plot. They usually consist of a monologue or dialogue sung with the natural inflections of human speech. As such, they rarely present a musical line and are usually characterized by fast-paced chatter in a monotone, aside question and answer dialogue to build dramatic tension. The two main types of recitatives are secco (that which is accompanied by continuo) and accompagnato (that which is accompanied by a full orchestra)

Aria
Arias are the more lyric, musical pieces of an opera. They release dramatic tension through an emotional melody, and are usually the ones audiences applaud and remember. Because of their inherent musicality, arias can effectively be sung and enjoyed outside the context of the operas they're written in, becoming familiar to people who perhaps have never heard the opera they're from. The most common form of aria is the da capo aria in the form A-B-A, meaning the solist sings one section, then a second, then returns to a variation on the first.

Overture
An overture is a mainly instrumental section heard at the beginning of an opera. Sometimes they are used to introduce melodies from the arias to follow. Generally, every act of an opera is opened with an overture, sometimes having interludes between scenes (called "sinfonias" in the Baroque era.)

Libretto
The libretto is the actual text of an opera, written by a librettist, creating characters and a plot to justify the theme of the music. It is divided in such a manner as to allow the composer to write various arias, recitatives, and overtures for the piece.

A not-so-brief history


Works Cited:
"The Enjoyment of Music" (ISBN 0393972992)
http://www.pbs.org/wnet/threetenors/time-content.html
Class notes from MUSI 1300 at the University of Texas at Arlington