Real Time or Reactive Systems are systems subject to time constraints. These are bounds which act as operational deadlines for processes.
While driving a vehicle, if we apply the breaks for a particular time(bound), the car stops. Similarly, if a bound is met the process stops.
A system may be classified as soft real time(deadlines met generally) or hard real time(deadlines met deterministically).
A soft real time may only lead to decrease in service quality while a heard real time may lead to complete failure of the system on meeting a bound.
Real time systems are used to guarantee that deadlines are met, are where throughput is not the major aim. These are used largely in medical equipment, missile systems, etc.