Punisher
In short
Frank Castle is a Marvel Comics antihero who uses lethal force to wage a private war against organised crime. He first appeared in The Amazing Spider-Man #129 in 1974.
In plain words
He is a former soldier who lost his family to criminals. Instead of following the law, he hunts down gangsters and criminals to kill them. He is known for his skull symbol and heavy use of weapons.
A simple example
If a criminal escapes a court trial, the Punisher might track them down and execute them personally.
Why it matters
He represents the extreme end of vigilante justice in fiction. His character explores the moral grey area between being a hero and being a murderer.
Easy to mix up
Do not confuse him with a standard superhero like Captain America, as he lacks a moral code against killing.