Frogs reproduce by laying eggs and the eggs hatch into tadpoles that grow into frogs. But not all frogs follow this steps, almost all frogs reproduce sexually, egg fertilization however happens outside the female's body instead of inside.
1 Additional Answer
Female frogs lay thousands of eggs in the spring. The male frog will climb on the females back and fertilize the eggs as they are being laid. The result will be lots of new little tadpoles in the water! You can find more information here: http://www.learner.org/jnorth/search/FrogNotes2.html