Echoes are trapped in a loop, okay? They keep replaying how they died over and over and over again, usually in the place where they were ganked. It's about as dangerous as a scary movie.
A death echo is a type of trapped ghost. The spirit is stuck reenacting its death over and over in a loop. Sometimes the spirit can be shocked into moving on, particularly by someone with whom the deceased has an emotional connection.
While investigating the Morton House, the home of Freeman Daggett, the Winchesters and the Ghostfacers encounter death echos in a few locations in the house. They eventually discover that Daggett stole corpses from the morgue where he worked as a janitor. As a result, their spirits are trapped in the house and forced to reenact their deaths for eternity.
After getting killed by Daggett, Corbett becomes a death echo, and is snapped out of it by Ed Zeddmore on whom he had a crush, when Ed tells him he loves him. After this, Corbett attacks Daggett, resulting in both their spirits being neutralized. It can be assumed that all the bodies were burned afterwards.
Episode Four - "CSI: Maggie"
While investigating the Grand Showcase Theatre for the spirit of Janet Myers, Ed theorized that she's a death echo trapped in the mirror, until Maggie discovered that her ghost was in the room with her, Harry, and Spruce.