Similar to the school bells that most of us heard during our school days, the New York Stock Exchange's (NYSE) opening and closing bells mark the beginning and end of each trading day. More specifically, the opening bell is rung at 9:30 a.m. to mark the start of the day's trading session. At 4:00 p.m., the closing bell is rung and trading stops for the day. There are bells located in each of the four main sections of the NYSE that all ring at the same time when a button is pressed.
Interestingly enough, the signal to start and stop trading wasn't always a bell. The original signal was a gavel, but during the late 1800s, the NYSE decided to swap the gavel for a gong to signal the day's beginning and end. When the NYSE relocated to its present location at 18 Broad Street in 1903, the gong was switched to the bell that we hear today.
A common sight today is the highly publicized events in which a celebrity or corporate executive stands behind the NYSE podium and pushes the button to ring the bells. Many consider this act to be quite an honor and a symbol of a lifetime of achievement. Furthermore, due to the amount of coverage the opening/closing bells receive, many companies coordinate new product launches and other marketing-related events with the day their company representative rings the bell. This daily tradition wasn't always this highly publicized. In fact, it was only in 1995 that the NYSE began having special guests ring the bells on a regular basis. Prior to that, ringing the bells was usually the responsibility of the exchange's floor managers.
(For related reading, see: What does "after the bell" mean?)