Restorations or "fillings" are done to remove decay, and replace the affected tooth structure. It is called a filling because new material fills holes that decay left. Now days most teeth are treated with bonded tooth colored composite resin fillings. Caught early enough, cavities can be treated easily and painlessly. If not treated decay can lead to tooth pain and/or infection, and the tooth would need root canal treatment or extraction.
Crowns are full coverage restorations that are used to cover a tooth that is likely to break, or is too broken down to be restored with a filling. They are most commonly done after root canal treatment, or when a large filling wears out. The larger the hole made by a cavity that has to be treated, the more likely a crown will be needed. Even after a filling is put in a large cavity, a tooth is more likely to break. Keep in mind that the jaw muscles are the strongest in the human body. Teeth are subjected to tremendous pressures. Crowns ride over the weakened tooth, providing strength and protecting the tooth against breakage. A broken or cracked tooth is a far more serious matter and much more difficult to treat. Crowns prevent this, as well as making for a nice smile.
This is an option for filling the space created by a missing tooth. It is formed to look like the missing tooth, and it takes its place in the mouth. The sides of a bridge use the two surrounding teeth for support, hence the name. A bridge replaces the missing tooth, both functionally and cosmetically. The materials used may be gold alloys, porcelain bonded to metal alloy, or all ceramic material. The choice of material depends on requirements for strength, wear, and/or esthetics.
There are different types of dentures, but they share their common function. They replace teeth that have become loose or been lost due to bone loss. When bone loss around the roots of teeth is great enough to loosen them or let them fall out, it's time for dentures. Relax. No one enjoys losing their natural teeth, but you can still eat and talk regularly.
Dentures are fitted to go over or around whatever teeth remain in the mouth, depending on the type. There is an adjustment period after dentures are placed in the mouth, and it can take some getting used to. But once accustomed to the dentures, all the normal functionality and appearance return and one just carries on as usual. Often implants can be used to further stabilize the dentures.