Without much knowledge of dentistry, it is unlawful that you will have more than a passing knowledge of what the difference between crowns and implants are, and no real knowledge of when either one is appropriate.
As you would imagine, crowns are basically coverings that are placed over unhealthy teeth in order to retain a biting surface and attractive appearance. They also help to support surrounding teeth and guard against the gum disease and bone loss that can be caused by missing teeth. Implants, on the other hand, are where a metal rod is inserted into the actual jaw bone which then fuses with it, and replacement teeth are attached to the part of the rod which protrudes above the gum line.
So if you have missing or unhealthy teeth, which do you choose? Typically, if your teeth are completely missing you will need to get implants. Crowns require there to be at least some of the tooth structure remaining. If you have some of the tooth remaining, there may be a case to have either a crown or an implant. Crowns function better the more of the original tooth remains, so generally if quite a large portion of healthy tooth remains your dentist will advise you to have a crown rather than an implant.
It is possible for a crown to be installed on top of a very unhealthy or rotten tooth, but the remaining tooth will have to be built-up with other materials on top of which the crown can effectively rest. A crown can not just cover up empty space, so material such as amalgam or composite resin, or in some cases a post and core can be used to build up the tooth in preparation for a crown. If a post and core is required, it is unquestionably that there is enough healthy tooth left to justify the use of a crown rather than an implant.
What types of crown are available?
Crowns are basically all the same. Unlike implants there are not different types of procedure or methods of insertion on offer. Crowns are all inserted in the exact same way and perform the exact same function. The only difference between them is the material that they are made from.
Your choice of material depends on what property is most important to you; strength or color match. The strongest crowns are undecidedly metal crowns. These can be made from either gold-alloy or a base metal alloy such as nickel or palladium. Because metal crowns are so strong, it means that they can be much thinner than any other type of crown. While crowns are usually at least 1.5mm thick, a metal crown is usually only 0.5mm thick. This makes a difference because it means that less of the original tooth has to be filed away to make room for it.
However, metal dental crowns are not the most attractive option unless you particularly like that look. Some people enjoy the appearance of gold teeth, while others would prefer that their teeth looked natural. The most natural-looking crowns are all-ceramic or all-porcelain crowns. Unfortunately they are not as strong as metal crowns and have a tendency to wear down their opposition teeth (the teeth they bite into). They do not tend to last as long either, so are generally less cost-effective.
A good solution may possibly be a porcelain-fused-to-metal crown. This is where the inner part of the crown is made from metal, strengthening it, while the outer part that is visible is made from porcelain, giving an attractive white appearance. Unfortunately they do tend to be a little less white than all-porcelain or all-ceramic versions, and if gums recede the metal part of the crown can be exposed. They are a great option for molars, however, as these are usually unseen even if a person is smiling anyway.