The crusades was a significant part in Europe from the 10-13th century, to retake the holy land and cities important to christianity. Acre, Jerusalem, Antioch, Cairo. But what has irked me is why did Islamic rulers hold most of Spain for more then half a millenium? Why was there no focus on driving Islamic rulers out of Europe THEN retaking the holy land?
Well it wasn't an either-or thing. There was a concerted, international effort to retake Spain from Muslim rule, sometimes involving crusading rhetoric and promises of indulgence from the Church. In fact, the city of Barbastro was retake by Christians (Spaniards and Normans) in 1064, and it would not be improper to think of it was the actual first crusade (or maybe Crusade Zero) as it was a papally-sanctioned expedition with "protecting Christendom" rhetoric supporting it.
As for why it took seven centuries...I'm not sure I can really answer that without taking all day, but the short version is that there was a lot of resistance by Muslims and nor always a lot of unity among the Christians. For a while there was a powerful caliphate in Córdoba, and there were multiple waves of reinforcements (who effectively took over) from Morocco.