Traditional crowns and zirconia crowns are made of the same basic material, porcelain. This is because porcelain is the material which has the closest optical properties to your natural tooth enamel.
Traditional porcelain is made out of a particular type which is called feldspar porcelain. All porcelain has two distinct phases in its properties. It has an aqueous phrase and a crystalline phase. In the traditional porcelain crown, the aqueous phase is 70% and the crystalline phase is 30%. The aqueous phase gives the porcelain aids aesthetics and optical properties in the crystalline phase is responsible for the mechanical properties.
In zirconia crowns, there is an increase in the crystalline phase to give the porcelain greater mechanical strength. Zirconia porcelain does not have good aesthetic and optical properties on its own. It has a white opaque appearance which obviously is not suitable especially for crowns in aesthetic work at the front of the mouth. In order to compensate for this, traditional porcelain is therefore used in a pressed form over the zirconia core.
This helps further increase the thickness of the crown, meaning that additional tooth removal is required. A traditional porcelain fused to metal crown requires as a minimum 1 mm of tooth reduction to accommodate the porcelain and another 0.5 mm for the metal. This gives a total of 1.5 mm reduction required.
A zirconia core requires 1 mm reduction but you have to take into account that you will be pressing a layer of additional porcelain which pushes the thickness back up to a normal porcelain fused to metal crown.
Furthermore, a recent advance has been that the zirconia core can come in different capacities from low, medium and high. The high capacity zirconia core can mask discolorations of a tooth underneath, such as large amalgam fillings or laboratory formed posts. This however impacts the overall aesthetic properties of the crown. If the tooth is not very discoloured, then you should go for a low or medium capacity core material of zirconia.
(Zirconia is just the generic name of the material. Zirconia porcelain is chemically different to the type of porcelain used in the normal traditional crown. As such, it can be known under different Brand names such as IPS e.max, Lava Plus, Suntech, Z-Crown, Zenith, Inceram, Cerec, IPS Empress, BruxZir, just to name some.)