Golems are humanoid creatures made of clay or wax, usually with round stones, marbles, or mirrors for eyes. They are brought to life with a spell written on a piece of paper placed into the body of the golem (typically the mouth). The earliest golems appear in the Talmud (a critical text in Judaism), and served to protect and assist their creators.
Clay golems are prone to hot, dry environments which can dry out their bodies, but they are very resilient in damp environments such as Northern Europe. Controlling a golem takes years of practice, though Dee demonstrates a mastery of this in The Alchemyst when he uses them to destroy The Small Book Shop, and later when breaking into Scathach's Dojo.