The Crusades resulted in an influx of goods, wealth and knowledge into Western Europe from the Byzantine Empire and the Muslim world. This influx was one of several factors that caused the Renaissance and Europe's shift from being a technological backwater to the most advanced region in the world.
The Crusades began in 1095 and arguably ended in 1291 with the fall of the last Christian stronghold in Acre. During this time, Crusaders brought back foods from the Holy Land such as dates, coffee, rice and sugar, items such as mirrors, carpets and compasses, and ideas such as algebra, irrigation and Arabic numerals. The knowledge of the Romans preserved by the Byzantines and various Muslim rulers spread through trade to Italy and by conquest to other parts of the continent as the Europeans pillaged the lands they visited.
One result of the Crusades was a major shift in the way that the European economy and political attitudes were structured during the 1100s and 1200s. Not only did these conflicts lead to a growing complicity between church and state, but opening the door to the Muslim world also led to shifts in the European economy that would open the door to new wealth.
Taking large armies from Europe all the way to the Holy Land required a massive supply line including outposts in the Middle East. The Italian cities of Venice and Genoa flourished by setting up lucrative colonies for trade in the Holy Land as well as parts of the Byzantine Empire that the Crusaders ended up capturing.
Before the Crusades, the church in the West, as well as the people, took a dim view of warfare. However, the calling of crusades indelibly linked the church with violence, and the necessary alliances that formed as a result brought religious and political leaders into alliances that would not always be healthy. While the Crusades did elevate the prestige and influence of the pope, they also created the precedent of confiscating possessions and lands from non-believers simply on religious grounds, and using local resources to fund faraway wars stunted development in Europe.