Here are the basics.
- The Crusades were first started in order to defend the Holy Land from Muslim invaders who had captured Jerusalem, and other territories, from Christians.
- The Byzantine (Eastern Roman) Emperor asked the Pope for help against the invading Muslim armies.
- Individuals did some horrible things during certain Crusades, which were never approved by the Catholic Church. This is not to make light of the evil acts, but to be historically accurate.
- Modern thinkers, esp. in the Enlightenment tradition have started to re-write the history of the Crusades from an anti-Catholic perspective. Thus, the word "crusade" has all kinds of baggage associated with it now of a religious zealot who will resort to any means to get their way.
A word of caution - we must be careful of reading our current view of the world into historical events, but at the same time we must learn from mistakes made in the past.
The Church knows that many bad things (e.g., killing of innocents, the sacking of Constantinople, etc.) were done during Crusades and has several Popes (including the Pope at the time - Innocent III as well as JPII) apologized for them.
Just as is the case when any individual, leader or not, in the Church sins - these things in no way invalidate the teaching of the Church. But, neither should we try to explain them away. Rather, we should be historically accurate and contrite if necessary for the sinful acts of others, even if they had a good initial intent.