Back in the year 911, Carolingian King Charles the Simple allowed the Viking Rollo to settle in what became known as Normandy.  In 1066 the Viking descendants invaded and conquered England at the Battle of Hastings. Thus Norman leaders, who were vassals to the King of France, even when they were the rulers of England. As the various regions or Normandy, Burgundy, and France changed hands, this complicated the relationship between those areas and the English Kings, and the English influence had begun to diminish.  By 1337, when the war began, the English concentrated their efforts to regain their Continental claims.  However, by the conclusion of the war in 1453, they had been driven out.  See more details at the link:

http://ehistory.osu.edu/osu/archive/hundredyearswar.cfm

Two of the most important factors of the Hundred Years War were the rise in royal power and the growth of nation-states. By uniting their subjects behind them during this time, the monarchs of France and England were able to build formidable armies... loyal armies. These standing armies became the great source of power behind the monarchs, essentially they became emperors in their own kingdoms.

The Hundred Years' War was actually several wars, with several different causes. The root was, as post #2 said, a dynastic dispute, but many other factors contributed, particularly a desire by both parties to control Flanders, which was becoming increasingly wealthy through the wool trade. Rebellions in Scotland and Wales also contributed, and the simple desire for loot on the part of particularly aggressive English nobles was a strong factor as well, particularly in the middle years of the conflict.

As for effects, it absolutely devastated the northern French countryside and its population, it stoked French and English nationalism and encouraged both kingdoms to develop more sophisticated mechanisms for raising revenues. It spelled the end of the dominance of mounted heavy cavalry, which had social implications beyond military tactics, since the heavy cavalry were knights. It introduced new technologies, including gunpowder.

The major cause of this war was a desire for power.  This was a dynastic war in which House of Valois and the House of Plantagenet both claimed the French throne.  Therefore, it was really just a war between two powerful noble houses as to who should control France.