In music pitch is the perception of the frequency of a sound wave. A sound wave can be conceptualized like a rope tide on one end while someone else oscillates it up and down on the other end. If you look at only one point you will see that the rope has a high point and a low point. If you pick one point to focus on, say the high point, you will find that it appears with regular occurrence. The speed at which this re-occurrence takes place is the frequency, usually measured in hertz.
The human brain receives input form sensory neurons in the ear that communicate the frequency of a noise to the audio cortex. The right hemisphere audio cortex specializes in discriminating differences between frequency. These subtle differences are what we hear as the pitch of the note, usually described as how low or high. The right hemisphere audio cortex is capable of differentiating pitches with in 1/4 of a semi-tone.
Much of the grammatical structure of human language is based on changes in pitch or tone. There is strong evidence that the same are of the brain that processes pitch for music also processes the pitch for language. There are people who can not differentiate between pitches very well. These people are referred to as having amusia or more commonly called "tone deaf." Those suffering from amusia can not detect a change in pitch greater than 2 semi-tones. This makes much of music difficult to understand and sound very similar. The interesting thing is that most human language pitch changes are in the order of 7 semi-tones, so those with amusia do not show any deficit in language. Some languages use use pitch to distinguish words; pitch when used thus is called tone, and such languages are called tonal languages.
Pitch also refers to a black tar-like substance that is made from sap plants or petroleum. It is used in shipbuilding.
Peretz, I., & Hyde, K. (2003). What is specific to music processing? Insights from congenital amusia. Trends in Cognitive Sciences, 7(8), 362-367.
Hyde, K., & Peretz, I. (2004). Brains That Are out of Tune but in Time. Psychological Science, 15(5), 356-360.
http://www.delosis.com/listening/home.html On-line program which tests for how well people can differentiate tones and timing.