The Crusades were fought from the end of the 11th century until the late Middle-Ages. In the main they were against the Islamic world of the middle-east and Spain. But others were proclaimed in Europe, most notably against the Cathar sect in the Languedoc of France.
Of all the Crusades against the Muslims only one was successful and that was the First Crusade. It was this Crusade that led to the founding of the short lived Frankish kingdom of Jerusalem, or Outremer(kingdom across the sea). One of the Crusades, the Fourth, only got as far as Constantinople, a Christian city. It proceeded to besiege it, finally sacking this magnificent city.
There was undoubtedly a religious context to all the Crusades and taking the 'Cross' was considered a holy undertaking for all Christians. But like all great ventures politics played a huge part. It is the reason why so many great men of the age took the 'Cross'. In the aftermath of the First Crusade, the leaders carved out mighty fiefdoms from their conquests. Duke Godfrey was proclaimed King of Jerusalem. Count Bohemond, the greatest warrior of the First Crusade claimed the Principality of Antioch and ended his days fighting against his Christian neighbors the Byzantine Empire. There were as many venal qualities in joining the Crusades as there were religious. A second son of a lord could gain immense riches as the careers of Bohemond and his cousin Tancred testify to.
One of the most famous Crusades was the Third Crusade, which was led by Richard the Lionheart , King Philip of France and the Holy Roman Emperor Frederick Barbarossa.This scored some notable successes at Acre and Jaffa, but failed in it's ultimate goal of retaking the conquered city of Jerusalem. The disunity of the Crusaders including the enmity between Richard and Philip played a large part in the failure. It is a matter of debate if Frederick Barbarossa, who drowned on the passage to the Holy Land, could have orchestrated a ultimate victory if he had lived.
A myth of this Crusade was the meeting of the great Saladin and Richard. It never happened. But such were the Crusades, a breeding ground for legends, misinformation, cruelty and bigotry. It's legacy lives on today.