I assume you mean the Knights (and others) who participated in the Crusades of the European Middle Ages. Crusaders were primarily knights (and therefore members of the nobility) who participated in the European effort to recapture Jerusalem from its Islamic rulers during the High Middle Ages. The word "crusade" was derived from the word crux, Latin for "cross." Those who participated in the Crusades were said to have "taken up the Cross of Christ." Many pinned a red cross to their tunics as a sign of their dedication. The Crusades were styled as something of a holy pilgrimage, so those who participated believed they would receive remission for their sins and forgiveness for any transgressions committed while on crusade--which were many. Crusaders were responsible for furnishing their own weapons and funding their own journey, which did prove burdensome, although a church decree forbade anyone from seizing their property for debt during their absence. Actual Crusaders are too numerous to mention, but a number of European monarchs participated in one or more crusades, including Richard II (Lionheart) of England and Frederick Barbarossa of the Holy Roman Empire.
In addition to the nobility who went on crusade, the First Crusade was preceded by a so-called "Peasants Crusade" in which a large number of serfs and peasants, inspired by a charismatic preacher known as Peter the Hermit attempted a crusade of their own which soon met with disaster. Also, a more interesting feature was the so-called "Children's Crusade," in which tradition says children marched toward Jerusalem in an attempt to reclaim it. More recent scholarship has cast doubt on whether the participants were actual children or were in fact peasants again, who were considered childlike.
There are numerous sources available you might consult; including Holy Warriors by Jonathan Phillips.
European Military and Religious-backed forces (many of whom were unprofessional fighters) who were urged by the Pope to retake the holy lands from the Muslims.
Crusaders were people who during the middle ages joined the Crusades to the Holy Land. They were a military force that the Roman Catholic Church sanctioned in its efforts to expel the Muslims and ecclesiastical heretics from the Holy Land and Europe. The time period for these campaigns was 1095 and 1291.