Most people probably don't need to be told who Lennon was. But as time passes, more and more people do. Lennon was a former member of the Beatles, one of the most influential rock groups in history, along with Paul McCartney, George Harrison, and Ringo Starr. The Beatles took Britain by storm in 1963 and toured the USA the following year, yielding the now iconic images of teenage girls fainting during their concerts, and teenage boys trying to imitate their moptop hairstyles.
The Beatles disbanded in 1970. Lennon eventually married and settled in New York City. Lennon is memorialized by a landscaped section of Central Park named Strawberry Fields, after one of his songs.
Chapman remains imprisoned in Attica.
Personal note: At the time Lennon was killed, I was only 10 years old. Although his death was a local story for me, I didn't really have an understanding of his impact until years later. My main exposure to the Beatles up until that point was the movie Yellow Submarine and an import copy of the Revolver album.
In the end, all Lennon wanted was peace for the world. Let's hope we find it one day.