The dog in the picture is running very fast. Other things that move fast are racing cars, planes and some animals. The fastest animal in the world is the cheetah. Another word for fast is quick. The opposite of fast is slow.
Something happens quickly if it happens in a very short space of time, or after a very short delay. Quick might also be another word for fast. The opposite of quick is slow. The squirrel in the picture is running very quickly.