Full Porcelain Crowns
These are very aesthetic, bonded crowns. They are mostly used for front teeth because they are the most natural looking type of crown and are often used in "cosmetic" dentistry.
There are many types, but they all have a common feature - no metal. They can occasionally break, but dental technology has advanced far enough to make them quite strong.
The 'gold' standard. Dental gold is about 60% gold alloy which is meant to match the hardness of the enamel of opposing teeth so both wear about evenly, an important trait. Gold does not tarnish or corrode and has some bacterial-inhibiting quality. Gold crowns are strong and will not break. However, gold crowns obviously are not considered aesthetic; they are gold colored. Usually, gold crowns are used for lower back molars because they aren't as noticeable.
Porcelain Fused To Metal Crown (PFM)
The most common type of crown and has a proven track record. PFM crowns are fairly aesthetic and they look like real teeth. However, the margins or borders may appear dark because PFM crowns have a metal substructure with layers of porcelain fired over the substructure. Porcelain is very hard, much harder than natural enamel and may cause excessive wear of the enamel of opposing teeth. Porcelain may break with extreme biting forces.
Stainless Steel Crowns
Stainless steel crowns, also known as 'silver crowns or caps', are a very common procedure used to restore 'baby teeth' (and occasionally permanent teeth) that have large caries or those that had a pulpotomy (baby root canal). Although most teeth can be filled with a white or silver filling material, stainless steel crowns are by far the most predictable and durable option to fix 'baby teeth' with large caries, large defects, or damaged enamel. This is particularly true in very young children that have caries at a young age, since it is preferred that the 'fixed' teeth last as long as possible.