The Italian Renaissance marks the period of transition between the 13th and 16th centuries in Europe. During this time humanism, a way of thought in which human values and interests prevail, was the norm in the scholarly world. Humanism was carried out through the arts quite successfully. The arts thrived during this period, and music was particularly affected by humanism. Vocal music during this time changed incredibly. While music of the early and middle medieval period tended to be chant-like, simple and homophonic, music of the Italian Renaissance sought to evoke the emotions of the listeners. With that in mind, composers strove to use melodic line and vocal technique to paint pictures in the mind of the audience, a technique called "word painting." So, if the text talked about a Jesus ascending to heaven, the melodic line would also ascend. If the text talked about the fiery furnace in hell, the technique would be much heavier and lower.
Composers also began using harmonies based on thirds and sixths, rather than fourths and fifths. These sonorities (3rds and 6ths) are the basis for music even now in the 21st century. Using a multitude of sonorities, both dissonant and consonant was thought by humanistic composer to evoke a variety of emotions in the listener.
Historical Significance:
The Italian Renaissance changed the way music was written. Composers used different sonorities, and sought to create music that was in itself beautiful, instead of music used strictly for worship. Styles of sacred music and secular music began to become intertwined and borrow from each other. Modes were used less and music became diatonic, which is the way music is written today.
This link is for Thomas Weelkes' madrigal "O Care, Thou wilt dispatch me." In it you hear word painting through the use of major and minor tonalities. When the text is sad the music is in a minor key, and much slower than the happy text, which is in major.
http://www.youtube.com/watch?v=ZobIf8KBcH4

This is a link to my all-time favorite composition, Tomas Luis Victoria's "O Magnum Mysterium." In it, you hear multiple examples of harmony based on 3rds and 6ths.
http://www.metmuseum.org/toah/hd/renm/hd_renm.htm

The Great Organ of Notre Dame of Paris
There has been an organ in Notre Dame since the cathedral was built. Music was an important part of worship for the faithful of Paris. Leonin, the first composer music historians recognize by name, and his student Perotin were both organists on the early and smaller version of the organ at Notre Dame. The current organ, know as the Great Organ, was built in the cathedral in 1401. The Great Organ has been remodeled and renovated several times over the last 600 years. Currently, the organ has five keyboards, one hundred ninety ties and eight thousand pipes, making it the largest organ in France and the most famous organ in the world.
The Great Organ is historically significant for several reasons. As I said above, it is the largest organ in France, and the most well-known across the planet. Additionally, some of the world's most famous organists and composers have performed here, including Couperin and Durufle.

The following link has some really nice listening examples of the organ being played.

http://mypipeorganhobby.blogspot.com/2008/11/organ-notre-dame-paris.html

This link has lots of pictures of the current organ, and details and information about the history of the organ.

http://www.notredamedeparis.fr/-The-Great-Organ-

Hildegard of Bingen (1098-1179), was an incredible woman during the Middle Ages. She was a teacher, abbess, author, mystic and composer, among many other distinctions.
Hildegard's music was significantly different from the music of her time. Where most "Gregorian" chant used a limited range, she used the entire range of high and low notes. Similarly, she incorporated leaps in the melodic line, when most chant used only stepwise motion. She varied the melody and had a flair for making a line more interesting than her contemporaries.
The following are some links to listen to some of Hildegard's music.

http://www.youtube.com/watch?v=LJEfyZSvg5c
http://www.youtube.com/watch?v=taanHO13WXE
http://www.youtube.com/watch?v=_STnAxZInrU

As a composer, Hildegard's music serves as one of the first examples of a move towards more elaborate type of composition. Her music is historically significant because, beyond being one of very few women composers of the time, she did not just follow in the footsteps of the composers before her. Instead, she created new and inventive ways to write music. Eventually, music in the church became much more ornate, using polyphony (more than one melodic line played/sung simultaneously) and without Hildegard's contributions, this might not have happened!