Generally speaking, a deal is an advertising inventory agreement between a buyer (advertiser or agency) and a seller (publisher or exchange). Depending on the specific type of deal, the terms, and implementation will vary.
Guaranteed deals are agreed upon between a buyer and seller up front. Because of this, many of the deal's settings like default targeting, budget and pacing, and insertion order settings cannot be changed. The default advertiser for a guaranteed deal can be changed, provided your user has the required permissions.