The "Blitz" is an Anglican shortening of the German word "blitzkrieg", which meant "lightning war." Germany had developed the means to coordinate its air and ground forces and annihilate any opposition, taking Poland and France by early 1940. Britain, who was alone in fighting Germany at this time, was not subjugated; once again, England being an island contributed to its unique history. In order to invade, Germany had to coordinate an amphibious landing, known as "Operation Sea-Lion." Troops and tanks couldn't cross the Channel and immediately attack, but aircraft could, until plans were complete. At first, the Luftwaffe attacked military and industrial installations, and Britain's Royal Air Force (RAF) retaliated on German installations. At one point, the RAF mistakenly bombed Berlin; Germany retaliated and concentrated on attacking London and other English cities (During and air raid in Liverpool in 1940, John Lennon was born!) Both sides suffered huge civilian casualties, and Germany lost more aircraft than could be replaced. However, the respite on military installations in England gave the RAF needed time to repair airfields and regroup and continue to mount a defense. The amphibious invasion never came, as Germany's military objectives changed when she went to war with the Soviet Union in 1941. Although Blitzkrieg was effective against the Soviets, historians agree that Germany committed a grave tactical error in attempting to subdue Britain by air power alone; not only did it not succeed, the German aircraft lost could not be replaced at the same level when they were desperately needed in the East, and were a contributing factor in Germany losing the Battle of Stalingrad, and thus the war.
The Blitz was a period during World War II in which the German airforce (known as the Luftwaffe) engaged in heavy bombing of Britain.
The time frame for the Blitz is generally given as September 1940 to May 1941.
The Blitz began at that point because the German Army had essentially conquered all of Western Europe. The only enemy they had left was Great Britain. While considering whether to invade Britain, the Germans bombed British cities in an effort to make it harder for them to produce war material and also in an effort to lower civilian morale (make people discouraged).
The Blitz ended when Hitler decided that an invasion wasn't going to work and that he wanted to invade Russia instead.