In 1095, the emperor of Bzyantium (Alexis I) sent ambassadors to Rome asking Pope Urban II for help. Alexis I needed help stopping the Seljuk Turks from invading Byzantium. The Byzantine emperor argued Byzantium prevented the Turks from invading Western Europe; therefore, it was in the interest of France, England, etc. to help defend Europe from invading armies from the Middle East.
Byzantium was the last remaining part of the old Roman empire in Europe; it was in decline and hadn't recovered from its defeat at the hands of the Turks during the Battle of Manzikert in 1071. Since this battle Turkey gradually expanded more and more into the Byzantine empire.
Byzantine's Emperor, Alexis I, hoped Christian Europe might send some knights and financial support. However, instead of a few knights his request led to a full scale invasion of the Middle East spanning spanning four centuries collectively known as the Crusades.
The Crusades were wars fought primarily over religion (but Christians also saw economic opportunities in the Middle East). The two major religions involved of course were Islam and Christianity. Although the majority of fighting took place in Palestine (or the "Holy Land") there were military campaigns fought in Europe itself against so-called "home grown" enemies of the Catholic Church.
OPPORTUNITY KNOCKS FOR THE POPE
The Crusades presented Pope Urban II with an opportunity: Europe in the 11th century suffered from non-stop war between rival Christian lords. If the Pope could get Christians to focus on fighting Muslims instead of one another he could bring peace to Europe. Even more important is the Pope could use these same lords to deal with his enemies in Europe, as well.
Although Christianity was the majority religion in Europe it wasn't the only religion. There were still parts of Europe where people hadn't convereted to Christianity. For example, in Ireland people continued to practice a religion called Druidism. In Sweden, the Norse gods Odin and Thor were still firmly in place in Sweden. Also, Poland and most of Russia remained outside of Christianity.
The main Christian kingdoms were located in Western (England, France, Spain) and Central (Holy Roman Empire). Powerful Muslim kingdoms were also present in Europe. Southern Spain was occupied by the Moors (a Muslim group); and Turkey, of course, was a region linking Europe with the Middle East. This is what Europe looked like just before the First Crusade was launched in 1097.
In the 11th Century, it was common for Christians to make pilgrimages to holy places like Rome, Constantinople or Jerusalem. Jerusalem was the most important city because it was in the Holy Land (or the birthplace of Jesus). In order to travel to Jerusalem, a person had to either sail the Mediterranean or travel along an overland route. Both routes were problematic.
Travelling by sea was expensive and dangerous (pirates). The journey over land was cheaper and also dangerous (robbers). So even at the best of times getting to Jerusalsem wasn’t exactly a safe or easy thing to do. The cost went up considerably with the rise of the Seljuk empire in Turkey. The Seljuks charged Christians a tax for safe passage through Muslim lands (and paying the tax never guaranteed you still wouldn't be killed).Thus, Christians were ready for war against the Middle East, in that, not only would invading the Holy Land end the constant war in Europe itself but it would open up trade and pilgrimage routes to Jerusalem.