Let's say you accidentally touch a hot stove. The pain receptors in your finger send intense impulses inward (afferent) to your spinal cord, which automatically sends impulses back out (efferent) to your skeletal muscles before the sensation even reaches your brain. You move your hand away before you feel the pain, and this is to minimize damage to your skin; the reflex arc is almost always protective.
To sum up in basic terms:
Afferent neurons send impulses IN to the central nervous system from sense receptors in the skin or internal organs, efferent neurons send impulses OUT from the CNS to the skeletal muscles. That's the basic difference; one group sends impulses in, the other sends impulses out.
I hope this helped!