Hip Hop refers to the African American music genre and the culture which surrounded it. Its time of birth was gradual, making it difficult to name a specific year. Most sources agree on early to mid 1970's.
The music its self evolved from earlier and other 1970's black American muisic, such as Disco, Funk and Jazz.
This music paired with certain Jamaican cultures led to the use of MC's talking over the music at live shows, which would later progress into what is now known as rapping.
Other important elements to the culture include grafiti, dance (b-boys) and especially Dj'ing. Early Hip Hop music was made by a DJ using his turntables, mixer and records. The arts of cutting and mixing were used to sample parts of records to create a whole new track.
In todays sense, Hip Hop has developed huge mainstreem success, and Gangsta Rap has become less about the music and more about comercialism, making money and 'pimpin bitches.'
There is, however a large underground Hip Hop following, which vows to stay true to Hip Hop. Madlib being a prime example of a true Hip Hop artist.