Taken from Wiki:
An oral ulcer (from Latin ulcus) is the name for the appearance of an open sore inside the mouth caused by a break in the mucous membrane or the epithelium on the lips or surrounding the mouth. The types of oral ulcers are diverse, with a multitude of associated causes including: physical or chemical trauma, infection from microorganisms, medical conditions or medications, and cancerous and sometimes nonspecific processes. Once formed, the ulcer may be maintained by inflammation and/or secondary infection. Two common oral ulcer types are aphthous ulcers (canker sores) and cold sores. Cold sores can be caused by the herpes simplex virus.
So these "things" are basically wounds, the usual remedy I hear is to rub salt in the wound. It works for some people apparently but for me, I rather NOT suffer the searing burning pain & fall to the ground writhing. Well roughly that's what happens to me *rolleyes*
You can prevent mouth ulcers by ensuring adequate oral hygiene, sometimes we can't prevent it if we accidentally bit/wound our mouth.
Some tips to alleviate the pain & to speed up the healing process:
I wish one day there will be a wonderful cure that will instantly remove these horrid things. Until that happy day, we've to be content with what we have :)