Dead Money is an entirely self-contained quest, totally seperate from the main quest. It has its own opening and ending slideshow.
It's very difficult and intended for high-level players (from level 20 up). Below level 10 it would be extremely difficult to finish.
The storyline ties into the Brotherhood of Steel. If you play through Dead Money, it unlocks new dialogue with Veronica. Your conversation with her will unlock a new perk depending on what you do.
The player is drawn there because they receive a radio signal advertising the grand opening of the Sierra Madre casino.
You have to assemble a team from three other companions there - a schizophrenic super mutant called either "Dog" or "God" with two different personalities (each has a different perk). Christine is mute for most the DLC. She is very damaged but also very intelligent. Dean is a lounge-singing ghoul with his own agenda. You have them on and off with you, but you have to interact with all of them throughout the add-on.
The quest will take around 4-6 hours for an average player.
The Ghost people were exposed to the radiation cloud surrounding the Sierra Madre. They are very mysterious, they don't talk, have glowing eyes. They don't die very easily - to really kill them, you have to decapitate them.