Christian knights from all over Europe came to Jerusalem to fight muslims because they revered Jerusalem as their holy land.
In this map it shows where the knights came from to fight for their holy land. They traveled great distances to fight the muslims to take back Jerusalem.
There were three main types of knights. One was the Knights Templar. The Knights Templar were established in the 1100s. They wore white clothes with red crosses that looked like the image below.
The second type of knights were the Knights Hospitaller. The Knights Hospitaller were founded in 1023. They were formed to protect poor and sick pilgrims in the Holy Land. During the Crusades they defended the Holy Land from the Muslims. These knights wore black clothing with a white cross.
The third type of knights were the Teutonic Knights. They were German knights that were once part of the Hospitallers. They wore black clothing with a white cross on the shoulder.
Many of the reasons the knights went to Jerusalem was to obtain land upon return. In medieval times the oldest son inherited the land from his father and the younger siblings did not get any. This upset the younger siblings so they created games to see who should inherit the land. One of these games was jousting. This game would leave only one alive to inherit the land. An alternative for these games was for the younger sibling to become a knight and get land for himself through crusades.
In total there were nine crusades and the Muslims won the holy war on Jerusalem because they had control of the city last. Out of the nine crusades they won five.
Most of the battles were fought near and in the holy city but some were fought in modern day Turkey.
The Muslim Calvary shot crossbows and had war hammers they would use to debilitate their enemy so the infantry could finish them off. They also carried a circular shield to protect them.
This video explains the perspective of the Muslims during the crusades. They did not ask for a war but the Christians decided to go and try to take the holy city back creating a war. The Muslims had been living there for a while so the Christians were basically kicking them out of their land.