Pain is how the body usually registers damage.
Pain hurts. Pain is generally bad.
Pain is good when brought to Mary-Sues. Or if one is a masochist.
Pain may be inflicted with a variety of instruments, including but not limited to:
- Blades (swords, knives, razors, etc.)
- Clubs
- Dendric strikers
- Guns
- Lasers
- Rocks
- Sporks
- Stale cafeteria food
- Tasers
- Vegetables
- Crucio (this particular example is unique to the Harry Potter canon, but equivalents exist in many universes)
Pain can be caused mentally by really, really bad fanfiction, especially bad smut.