Periodontal: Refers to the tissues and structures around the teeth. Generally this area is concerned with gum diseases that can then lead to tooth  loss.

Prosthetic Dentistry: The word prosthesis is defined as  an artificial device used to replace a missing body part, such as  a limb, tooth, eye etc.

In prosthetic dentistry there are  several subcategories:
Crowns- Like a cap that covers the top of the tooth and down the sides to hold it all together.
Bridges - these fill the gap for one or more teeth that are missing and are anchored to one or more adjacent teeth.
Dentures - Partial or whole this is a removable appliance with plastic or porcelain teeth.
Implants - Titanium screw surgically implanted into the jaw bone.

Pyorrhea: A more extreme form of gum disease that includes teeth loosening, bleeding gums and bone being resorbed (dissolved).

Root Canal: This is a dental treatment that involves removing diseased tissue from the tooth root and replacing it with an inert material.

Teething: This refers to the normal process  children go through as the teeth break through the gum for the first time. Also known as eruption or cutting.