The Crusades of the late eleventh to the thirteenth century had a substantial effect not only on Western Europe but also on the Byzantine Empire and the Middle East, as well as the relations between the three. The impact of the Crusades was felt even further afield if we count among them, as many historians do, the attacks of the Teutonic Knights of the Livonian Order on Russian principalities of Pskov and Novgorod. In any case, the Crusades were an expansive phenomenon, and their effects were far-reaching and long-lasting. The brief comments below might suggest productive avenues to pursue to delve deeper into some of those effects.
The Crusades provided some benefits for Western Europe: they led to the development of the iron and shipping industries, the papacy grew in power and prestige after having spearheaded the wars and gained new adherents, the autonomy of women grew since men were away at war, and Europeans borrowed knowledge and skills from the societies they encountered in their campaigns.
While Europe benefited in some ways, the Crusades also brought problems. The slaughter of civilians created an image problem for Western Christians (Catholics) in the East, not only among the Muslim population but also local Christian and Jewish populations. During the First Crusade, the Western forces slaughtered Jews wholesale once they reached Jerusalem.
The Crusades also deepened the rift between the papacy and the Orthodox Christian world. If the first three crusades were marked by misunderstandings and tensions between the Western and Byzantine rulers, the brutal sack of Constantinople during the Fourth Crusade, according to the judgment of some historians, made the Great Schism of 1054 irreversible and permanent. Catholics, the Byzantines concluded, were certainly no brothers of the Orthodox.
Amin Maalouf, in his book The Crusades Through Arab Eyes, claims that the Crusades exposed the weaknesses of the Arab world. By that time, the leaders were foreigners: Turks, Armenians, and Kurds. Arabs had lost control of their own destiny. Military losses and a trail of problems in the wake of the Crusades revealed the absence of stable institutions and the lack of unity within Islam. Whereas western openness and borrowing from the lands they invaded contributed the Renaissance in the West, the Arab world, viewing itself as having been victimized by outside brutality, closed itself off and began to stagnate.
Any of these topics can be productively explored. There are fairly abundant primary and secondary sources on the Crusades and their impact. See the link below to get started.
One major social effect of the Crusades was that going on a pilgrimage opened people's eyes to the world around them. Many who went wrote travelogues in order to tell other travelers what to expect. The upper classes who participated wanted to have the food that they experienced while in the Holy Land; this led to creating an even larger market for spices.
Economically, the Crusades were quite important for Europe. Conquering Europeans brought back relics from the Holy Land; relics were quite important in order to stimulate donations for churches. The Vatican was also able to convince monarchs who could not go on Crusade to at least donate money for the effort. This further enriched the Papacy. Crusaders also helped drive a growing interest in Eastern goods and spices, and silks became an important trade byproduct of going on Crusade. Even after the Crusades, these continued to be so important that Europe began to consider seaborne routes to the East. Venice grew quite rich off the Crusades, as the state was one of the few who specialized in maritime warfare and travel to Byzantium.
The Crusades were politically important as well. The Crusades allowed Europe to briefly claim land in the Middle East before it was retaken by Muslims. The leaders of Europe quit fighting each other long enough to fight; this was quite important, as Europe experienced nearly constant warfare during that period. Crusaders sacked Constantinople during the Crusades, thus weakening a vital bulwark against Muslim expansion toward Europe.
The Crusades had many effects on Europe. They demonstrated that, even though Europeans still fought petty wars, they still viewed themselves as members of Christendom when faced with fighting other religious groups. The Crusades were also one of Europe's first exposures to trade goods from the Middle East. While they ultimately failed, the Crusades changed the European economy and social structure.
The Crusades had a major effect on commerce, as the crusaders often brought goods, such as silks, perfumes, ivory, pearls, and spices, across the Mediterranean to the ports of Italy. This trade stimulated a demand for eastern goods into Europe, and trade with the East became increasingly important. In addition, the Crusades created an immense demand for ships to transport the crusaders to the East. Italian cities such as Genoa and Venice became wealthy through trade to the East. Politically, the Crusades resulted in the weakening of feudalism and the growth in royal power. Many of the knights who were important in propping up feudalism went on the Crusades. Their absences or death resulted in the collapse of many estates, which were then transferred to kings. The Crusades also resulted in the intellectual and social advancement of Europe, as knights returned with knowledge of science and literature that was far more advanced in the East than in Europe at the time. As a result, the Middle Ages eventually gave way to a rebirth in learning and exploration known as the Renaissance.