Negotiation games explicitly involve and encourage making deals and alliances with other players and backstabbing when appropriate. Winning is rare without participating in these deals. Unlike cooperative games, Negotiation games are still largely competitive, while granting players certain times to make mutual agreements through discussion.
Negotiation also includes bribery, such as is used in Santiago or Power Struggle. In these situations, the negotiation is in the form of a naked offer, which can be accepted or declined.
For an overview and discussion of popular negotiation games, see: Negotiation Games: Your favourites and why
Negotiation games fan