Tracing back the history, classical music is divided into six different periods.
The longest and most distant era of musical history is the Medieval Era, otherwise known as the Middle Age Period. During this period, the Church was the center of social life, learning, and the arts. In the early centuries of the Christian Church, Saint Gregory, who was Pope at that time, arranged and collected choral works in the form of chants, called the Gregorian chant. This is music composed with a single melodic line. Polyphony, on the other hand, is music with two or more melodic lines in one harmony. The musical notation used is neumes, which is very different from what is used now.
In the Renaissance Period, composers were more recognized and more expressive than in the previous era. The collection of instrumental music became more varied along with the invention of new musical instruments. Secular music also became more common which now included other forms like motets, madrigals, and songs. Polyphonic lines were used in a different way. The development of music during this period was mainly in Rome since the Church occupied a dominant position in society. Most of the works by the composers are in Latin.
The birth of the opera occurred during the beginning of the Baroque Era. Orfeo, the first opera, was composed by Claudio Monteverdi. Musical instruments became more popular and became the main sources of music. Music in this period was richly textured, highly metaphorical, and was used to express human emotion or depict natural phenomenon. The Four Seasons by Antonio Vivaldi is a good example. During this era, concerto became the most popular music genre. Music in the Classical Period was rather curious since very little development was done to alter the musical language other than the abolishment of polyphony. The orchestra was born during this period. This was also the era of famous composers such as Mozart and Beethoven. Musical writing was rather simple but musical harmony was developed.
The Romantic Era was more reminiscent and music was expressed with such spontaneity, fantasy, and sensitivity. The color of sound became an essential tool for human expression through music. This period also became the golden age for composers, immortalized by such greats as Franz Schubert and Peter Tchaikovsky.
Around 1900, the face of music changed, ushering in new possibilities. Music of the 20th Century is a continuation of the Romantic era, but more reactive to romanticism. Popular and classical music became distinct from one another. Composers began to experiment with the different elements of music, creating their own borrowed chords. The advancement of technology also created a great impact in the evolution of music in this period.
Information from http://www.zzounds.com/edu--musichistory