Zionism is many things to many people. At its base, Zionism is the belief in the necessity for a Jewish homeland or state. As an actual movement, Zionism started in the early 20th century. Jews were increasingly finding it difficult to live in Europe and the desire to end the diaspora was growing. There was much debate over where to form this new Jewish homeland. Many argued for South America. One wealthy American Jew actually bought several thousand acres in upstate New York (on the Canadian border) and proposed it as an option. The draw to the 'historical homeland' of Palestine was great. After WWI the state of Palestine was disorganized. There was little nationalistic identity, and during the breakup of the Ottoman Empire, Jordan, Syria, and Lebanon all made attempts to incorporate the territory that became Palestine. Wealthy American and European Jews began to buy land from wealthy Arabs, and then encouraged poor European Jews to migrate. Many Jews who already lived in Palestine argued against this, believing that such migration would cause class strife that would manifest itself along religious lines. As poor Arabs lost their jobs working the farms/orchards/etc, this is precisely what happened - what started as violence over jobs quickly developed religious overtones on both sides and the animosity intensified. After WWII there were millions of displaced Jews who had no where to go. To make a long, complicated story short, it was decided to go to Palestine and create the state of Israel. The UN/major world powers supported this action (or at least didn't actively oppose it), and Palestine lost most of its territory. The next 50 years saw several attempts by the Arab nations to retake Palestine (supported in large by the Soviet Union). All of these attempts failed to achieve real success (there were some minor gains from time to time) thanks to the support of Western nations (France, US, England) and the intense desire of the citizens of Israel. Eventually the conflict has devolved from open warfare into guerrilla and/or terrorist warfare.
Most years since 1948, Israel, Palestine, and the Zionist question have made world headlines due to the violence and strife. Some have argued that Israel destabilized the region. Others state that it is only fitting and proper that there is Jewish state, pointing to the numerous pogroms in Europe and the Americas over the last 600 years. Some argue that Israel has its right to exist but must treat the Palestinians properly. The variety and complexity of the issues surrounding Israel and Zionism is enormous. It is an issue that has been at the forefront for generations and will continue to be for decades to come.