Negro is an obsolete term for a person of African ethnicity or racial characteristics. Through the middle of the 20th century, it was commonly used in the United States, and was generally considered a well-intentioned and polite term. As an example of this, it was used 15 times in Martin Luther King Jr.'s I Have a Dream speech. But during the Civil Rights movement of the 1960's the term fell out of favor, and has generally been replaced by "black person" or "African-American".
The reasons for this change are a bit subtle. The point is that "negro" is a noun, whereas "black" is an adjective, used with the noun "person". The use of the word "negro", just by itself, therefore suggested in a subtle way that the subject was something other than a person. Reading things that were written prior to around 1960, one can sometimes see the word used in a way that slyly makes the distinction between a "negro" and an ordinary "person". This difference may seem trivial and picayune now, but it was felt by many to be very real at the time.
The word comes from the word for "black" in the Latin-based languages, such as Spanish, where the word is simply an adjective meaning "black". The names of the nations of Nigeria and Niger are both widely assumed to come from this root, but the etymology is somewhat obscure.
This word is not to be confused with the deliberate mispronunciation that has historically been used in an explicitly racist and hateful way. That word is simply unacceptable in modern society, and, when it must be described, is referred to as the "N word".
Although the term "negro" is obsolete in modern society, it, and the similar term "colored person" still appear in the names of two well-respected and long-standing organizations—the United Negro College Fund and the National Association for the Advancement of Colored People. The term "Negro Spiritual" is also still used for a musical style that grew out of American slave society.