Opera is more of an art form than just a music genre, it's an art form where singers and musicians get together to perform a dramatic and generally theatrical work, refereed to as an "Opera", this would combine a text/story line with a musical score.
Opera is usually part of the Western Classical music tradition, although it has been combined with many other genres of music to create things like the Rock Opera, this is where more contempory instrumentation and vocal styles are used to create the musical score and the vocal lines.
Standard Opera would incorporate many of the elements of spoken theatre, including acting skills, scenery sets, costumes and sometimes dance. The Opera performance is usually given in an Opera house or theatre, and is accompanied by an orchestra or smaller musical ensemble.
Opera grew out of Italy towards the end of the 16th century and was often put together by what was seen as the rebels of their day. One of the earliest known Operas would be Jacopo Peri’s lost Dafne, produced in Florence around 1597 and soon spread through the rest of Europe:
Heinrich Schütz in Germany, Jean-Baptiste Lully in France, and Henry Purcell in England all helped to establish their national traditions in the 17th century. However, during the 18th century, Italian inspired Opera continued to dominate most of Europe, whereas France was attracting foreign composers such as Georg Friedrich Händel.
One of the first main changes and rebellions came in the shape of Christoph Willibald Gluck who reacted against what he saw as its artificiality and created “reform” operas in the 1760s. In more recent years, the most renowned figure in Opera grew out of the late 18th century and is a well known figure;- Wolfgang Amadeus Mozart, he's most famous for the Operas, "The Marriage of Figaro", "Don Giovanni", and "Così fan tutte", as well as "The Magic Flute", which was a landmark in German traditional Opera.
The early part of the 19th century saw composers experimenting with the "bel canto" style, these included "Gioacchino Rossini", "Gaetano Donizetti" and "Vincenzo Bellini" all creating works that are still performed today.
Those early days of the 19th century also saw the advent of "Grand Opera" which typified works by the likes Meyerbeer.
The mid to late 19th century is considered by some a golden age of opera, with Opera being led by Richard Wagner in Germany and Giuseppe Verdi in Italy. This ‘golden age’ developed through what was known as the "verismo" era in Italy and contemporary French opera through to Giacomo Puccini and Johann Strauss II in the early 20th century.
During the 19th century, parallel operatic traditions emerged in Central and Eastern Europe, particularly in Russia and Bohemia. The 20th century saw many experiments with modern styles, such as serialism (Arnold Schönberg and Alban Berg), Neo-Classicism (Igor Stravinsky), and Minimalism Philip Glass and "John Adams". Philip Glass is even cited by some Gothic and some Industrial acts as an influence.