Complex sentence: Whereas a "stopwatch" counts upwards from zero for measuring elapsed time, a "timer" counts down from a specified time interval, like an hourglass, often ringing a bell or buzzer when the time interval has elapsed.
Split into simpler sentences: Whereas a "stopwatch" counts upwards from zero for measuring elapsed time, a "timer" counts down from a specified time interval, like an hourglass. It may ring a bell or buzzer when the time interval has elapsed.