UART frame has 1 start bit, 8 bit payload data, 1 or 2 stop bits. Frame structure and baud rate is agreed by both transmitter and receiver for successful data transmission. Start bit is used to synchronize and tell receiver that frame is started. Why there is a need of stop bit per frame, since all data frame to be transmitted will always be 8 bits (frame will always end with 8 bits)?
It's a holdover from the days of electromechanical teletypewriters, when the time was used to release a clutch in the electromechanical transmitter mechanism. It was carried forward into fully electronic UARTs, perhaps because the original electronic designers didn't understand electromechanical teletypewriters.
Here's a discussion of how the old teletypewriters worked: http://mysite.du.edu/~jcalvert/tel/teletype.htm