Calculus is used when you want to use the rate of change; for instance, if you know the rate of the change in the temperature in a room (along with knowing other variables), you can predict the temperature (and the rate of change of temperature) at a previous time or at a future time.
It is used to predict instantaneous rate of change.
For example, it is not hard to calculate you average speed over a period of time: speed = (final location - initial location) / (final time - initial time).
Specific example: you are on an interstate and you pass mile marker 100 at 1:00. Later you pass mile marker 170 at 2:00. Your average speed was (170 - 100) / (2:00 - 1:00) = 70 miles / 1 hour = 70 miles per hour.
However, how do you calculate the speed of the car at the instant it drove off the road; at the instant it left the road, its final location was the same as its initial location (one point, so the distance it travelled that instant was 0 feet) and its final time was the same as its initial time (so its time was 0 seconds). Placing these values in the speed formula, you get 0 / 0. Calculus gives ways to handle these "indeterminate" expressions.
Calculus also gives ways to calculate areas and volumes (these have uses in solving problems that really have nothing to do with area and volume in the usual meanings).
Among other uses, scientific notation is used to express very large and very small quantities:
98 000 000 000 becomes 9.8 x 10^10 and 0.000 000 000 07 becomes 7 x 10^-11