I don't know much about mosques either, except they're all built in the direction of Mecca and have a vast covered hall for the faithful to pray in... In the present case, the hall in question is pillared and vaulted, with double arcs "in the likeness of a grove of palm trees in an oasis". It was the biggest (seemingly) muslim building in Europe in the Middle age, and has been preserved because the cathedral was build inside it, although Charles V of Spain later complained that the original monument had been defaced (which says something about open-mindedness, btw!).
A general view is available at the wikipedia page: