Metal Crowns have a core of metal onto which a tooth coloured porcelain is fired.
These crowns typically are held in place by a slight taper in the preparation design, and a cement. Preparations of these types of crowns mean tooth structure is removed all the way to the gum margin that sometimes results in the strongest part of the tooth been weakened.
Metal Free Crowns are made up of porcelain (IPS e.max Ceram)
IPS e.max is a ceramic material which combines strength and beauty unlike any other dental restorative. With the new bonding systems available, porcelain can be reliably bonded to the tooth structure, allowing a more conservative approach to preparing the crown without having to cut down to the gum level. These types of crowns are also known as overlays - and your dentist may use these two terms interchangeably - crown/overlay.