Music history is a subfield of musicology (which is the study of music) which is concerned with various elements of music such as composition, performance, and music criticism over the course of time. Music history examines the life, work and historical context of composers, the developments of musical styles and genres, and what part music has played in specific societies or groups of people. Music history also examines the relationship between words and music, the analysis of musical texts, and the biographies of musicians.
Simply put, music theory is the study of how music works and the various elements of music (including but not limited to rhythm, melody, structure, harmony, and form). Music theory examines techniques and patterns in music across genres and historical periods.
Music is the art or science of combining instrumental sounds and or vocal sounds to produce a beauty of form, an expression of emotion, and harmony. It is the arranging of sounds to create rhythm, harmony, and a unified composition--usually designed to be aesthetically pleasing and emotionally evocative.
Humans have been making musical instruments since prehistoric times. A flute has been found which was believed to have been made more than 40,000 years ago. Flutes and string instruments have been found in during archaeological explorations of the Indus Valley, and references to Indian classical music are found in the Vedas. A group of musical instruments dating back to approximately 7000 B.C. were discovered in China. The oldest surviving notated piece of music, written on a clay tablet, dates to roughly 1400 B.C. The origins of Western classical music come from the music of Ancient Greece, where music was part of education and Greek music theory was developed.
Music had made great strides by the 1600's, which ushered in the period of Baroque music (1600-1750). During the Baroque music era the earliest operas were composed and music was written for ensembles which included string instruments, pipe organs, choirs and harpsichords. The Classical music period (1750-1800) saw the further development of Baroque forms such as the sonata and the concerto. The symphony was invented during the Classical music period. Composers of the Romantic period include Beethoven, Tchaikovsky, and Mahler, who used more complex techniques and dissonance.
The 20^th century saw more people than ever listening to music thanks to the radio and phonographs (and later records, tapes and CD's). Igor Stravinsky, Arnold Schoenberg, and John Cage are among the composers of 20^th century art music. The 20^th century has also seen the ushering in of jazz, rock music and its subsets (heavy metal, punk rock, progressive rock, etc).