What armies did the Crusaders fight? What was their religion?
In the more traditional sense of the term "Crusaders", they fought with Muslim civilizations that had moved into and were expanding through the Holy Land.
"The Crusades" is a term that refers to a series of military campaigns that were called for by the Pope and in which participants would receive indulgences from the Church (which basically meant that fighting in the Crusade would constitute and act of penance that would absolve you of your sins. And by absolve, we're talking complete and total absolution - people without this absolution would have to suffer through a period of time in Purgtory and basically suffer for their sins. And since the difference between Hell and Purgatory was only the amount of time one stayed there, indulgences were something very much desired). The aim of Crusade was to purify the world - at that time it was a stated intent of the Papacy to bring Christianity to the world and have the Pope as the head of it all.
A Crusade could be one against a foreign enemy - and that is the situation this question is asking for. But it could also refer to dealing with internal divisions within the Catholic Church.
Because of the fairly broad definition of a Crusade, there is some debate as to what exactly was one and what was not one and how many there were. However, the more common usage of the terms Crusade refers to a series of campaigns against the Turks and Saracens, civilizations that had expanded into the Holy Land (present day Israel and surrounding countries) and were expanding further towards Constantinople (now present day Istanbul). Those civilizations practiced Islam.
The Pope called for forces to retake the Holy Land from the invaders and to open ways for Christian pilgrims to reach there safely. (He also hoped, through providing aid to the Eastern Orthodox Church, to reunite the Christian church under one banner - with the Pope at the head of it).
The Crusades produced mixed results. The First Crusade was the only crusade to actually retake the city of Jerusalem, in 1099 AD - defeating the Muslim and Jewish defenders of the city who worked together in the city's defence and then massacring the inhabitants and looting the city. Jerusalem was retaken by the Muslim general Saladin in 1187. No other Crusade would retake Jerusalem again.
What other crusades did do, however, was to massacre Jews in the Rhineland (now in present day Germany), sack and loot various cities (including the 4th Crusade that couldn't raise enough money to pay the ships transporting the Crusaders, so they sacked Constantinople twice, thus ending the Papacy's desire to incorporate the Eastern Orthodox Church, based in Constantinople, under the Papacy's control), and loot and pillage the countryside they traveled through to get to where they were going.
The Crusades highlighted the idea of Christian purity and helped to spark the Inquisition.