What is the definition or description of: canker sores?

Mouth Sore: In general terms, they are small shallow ulcers that appear in the mouth typically lasting about a week. They are often painful.

Herpes Simplex: Almost everyone is infected with this herpes virus. The first episode is usualy in early childhood. It is dramaticaly different from all the later events. The virus is always present. When your immune system is weakened (stress, disease, almost anything) the viri end up stronger than your immune system. They do their job, and you get "canker sores.".