The term applies to the often bloody struggle between Christian and Islamic faiths for primacy over the spiritual treasures of Jerusalem and the Holy Land, which dates back to the 11th Century.
The Crusades began in 1095 after Seljuk Turks took control of Jerusalem and began restricting access to Christian pilgrims. Pope Urban II called for a Christian army to retake the city from its Muslim rulers - sparking a 200-year period in which parts of the Holy Land repeatedly changed hands, until the last crusade ended in defeat for the Christians in 1291.
Urban II saw the Crusades not only as a way of freeing the Holy Land, but also of extending the influence of the Roman Church into the Byzantine Empire - today's Balkans and much of Turkey - through which the army would have to pass before reaching Jerusalem.
Glory and redemption
The first Crusaders, who set off in 1096, were a motley, and ultimately unsuccessful, bunch - peasants, from France and Germany, spurred on by the prospect of more freedom. Having pillaged and killed their way across Europe, they were vanquished by the Turks.
Six months later a more professional army, comprising French and Norman knights, set off. They successfully stormed Jerusalem in July 1099, making it one of four "Crusader states" in Syria and Palestine.
Serious trouble flared again in the early 12th Century when the Muslims took one of the other Crusader states in 1144, prompting the Second Crusade. However, its armies were almost wiped out in Asia Minor.
Knights held much power in the Crusades
Things stepped up apace when the Turkish armies came under the command of Saladin, a Kurd, considered the greatest Muslim leader of the time. He reconquered Jerusalem prompting the Third Crusade, jointly led by Britain's best-known Crusader, Richard I or Richard the Lionheart.
Although Richard and co failed in their prime goal - to snatch back Jerusalem - they defeated the Muslim forces at nearby Acre and reached a peace with Saladin over Christian access to the Holy City.
The Fourth Crusade, which started around the turn of the 13 Century, was a bit of a bungled affair, which ended with the warriors being excommunicated by Rome after they decimated the Catholic port of Zara on the Adriatic and fought Christians in Constantinople in 1204, destroying valuable treasures.
Things reached another low with the Children's Crusade of 1212, led by 12-year-old French peasant boy, Stephen of Cloyes, and a 10-year-old German boy, Nicholas. They mobilised an estimated 50,000 children between them but both child armies were betrayed and taken into brothels before leaving Europe or sold as slaves at Alexandria.
Another failed Crusade - the Fifth - followed, before Christians decided to switch tactics and try negotiation rather than brute force. Success! The peaceful Sixth Crusade in 1228 restored Jerusalem to the Latin world and a 10-year truce was signed. But things fell apart when Muslims later reoccupied the city, prompting yet another Crusade in 1248. It collapsed when its leader, Louis IX of France, was captured. Two later Crusades both failed and the Turks took the last Christian stronghold in the region, Acre, in 1291.
So how are these turbulent events viewed today, with the hindsight of several centuries?
Muslims do not single out the Crusades as a defining event in their history, according to historians. The wars have always been more of a western European obsession, with figures such as Richard the Lionheart held up as icons.
"For most Muslims the Crusades were something they won but just another invasion among many in their history," says Dr Jonathan Phillips, author of The Fourth Crusade and the Sack of Constantinople.
It wasn't until recently that the Muslim world started to take a renewed interest in the Crusades.
Syrian castles became Muslim strongholds during the Crusades
Muslim scholars have been returning to historical texts and important documents are being published in English, including the diary of Saladin's secretary. Saladin has also been revived as an iconic figure. The Muslim leader has been cited by Saddam Hussein and al-Qaeda as an inspiration.
"There is a straight line for propagandists to draw," says Dr Phillips. "Saladin drove out invading westerners and there are parallels with the current situation in places like Iraq that resonate with the average Muslim, especially after George Bush's ill-advised use of the word 'crusade' when launching his War on Terror in the wake of 9/11."
In northern Europe, the crusades crashed waves of violence upon the Jewish communities. Jewish people felt the brunt of the religious fervour that sent the Crusaders into the Holy Land, says Prof Anna Sapir Abulafia of the University of Cambridge.
Not only were they the most visible non-Christian community, says Prof Abulafia, but they also suffered because they generally weren't riding off on crusade themselves and weren't "part of all this non-Christian propaganda and hype".
In places like York, there was a massacre of the Jewish community in 1190.
"If you start preaching a Crusade and have accepted violence against non-Christians... that then evokes all kinds of violence against Jews."
The New Jewish Encyclopaedia calls the crusades a "prolonged and bitter ordeal" for the Jewish community, saying "thousands of Jews perished, and entire Jewish communities were wiped out. To this day, the Jewish liturgy contains prayers commemorating the martyrs of that dreadful period".
The crusades made the news in 2000 for a simple reason: Pope John Paul II apologised for them. Sort of.
The pontiff made a plea for forgiveness of the past sins of the Church, saying,"We are asking pardon for the divisions among Christians, for the use of violence that some have committed in the service of truth, and for attitudes of mistrust and hostility assumed toward followers of other religions."
It's a commonly held view amongst moderate Christians that the Crusades are a shameful part of the religion's history, experts say.
However, some more conservative Christians side with the belief that The Crusades were a series of defensive wars against Islamic aggression.
Former presidential candidate in the US Pat Buchanan has said: "Now, we must also be ashamed of Crusades launched to recapture, in the name of our Lord, the Holy Land seized from Christendom by the armies of Islam."