Why was Britain able to avoid being invaded by the Nazis in World War II?

The German army moved across Europe in an attempt to
subjugate Europe. First they captured Poland; then they took the Netherlands, Belgium,
Denmark, Norway and France. Italy, an ally of Germany, attacked Greece and the Balkins,
and in March and April of 1940, Germany occupied Yugoslavia and
Greece.

However, though some online sites mention that the
German did try to invade England by sea, there seems to be a
greater consensus that the Nazis never did so. The reasons given
were the strength of the British Royal Navy and England's ability to resist air attacks
by the Germans.

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Hitler did not attempt to invade England on
account of the strength of the British navy, and England could not be put down by air
attacks. England won "the battle for England" in the
air.

That is not to say that
Hitler did not entertain the idea of trying to invade England.
During 1940-41, Hitler did, in fact, prepare for a British
invasion, but plans ended when:

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Hermann Göring's Luftwaffe [German Air Force]
failed to gain air superiority over the Royal Air Force in the Battle of
Britain.

The German's air
campaign against Britain occurred during the summer and fall of 1940. The objective was
to:

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...gain air superiority over the Royal Air Force
(RAF), especially Fighter
Command.

England's Prime
Minister, Winston Churchill, delivered his famous speech which gave this decisive battle
its name:

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The Battle of France is over. I expect the Battle
of Britain is about to
begin...

The Battle of
Britain was fought entirely in the air. The Germans targeted areas they felt would force
the English to surrender, including shipping convoys and centers, RAF airfields,
aircraft factories, and finally, ground infrastructures (roads, water supplies, etc.).
The Germans then shifted their attention to bombing "areas of political significance,"
using "terror bombing tactics."

Germany's failure to force
England's hand is considered the Nazi's first major defeat, as well as a critical
turning point in the war. Had the Germans been
successful...

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Hitler might have launched Operation Sea Lion,
an amphibious and airborne invasion
of Britain.