An orthodontist has specialty training in the treatment of bite problems. Most people are very familiar with orthodontistry's braces and retainers.
A periodontist has specialty training in the treatment of advanced gum diseases / problems. They are involved in gum diseases that often require surgical treatment.
An oral surgeon has specialty training in the surgical treatment of the mouth. They are commonly involved with treatment for difficult extractions and many other diseases in the mouth and jaw.
An endodontist has specialty training in the treatment of root canal problems. An endodontist is required for very difficult, and re-treatment type root canal cases.