A Private Match is a player-defined variant of standard Game Modes.
Private matches essentially allow friends to play regular variants of the standard in-game multiplayer game modes. They are primarily used for custom games with special rules that would be difficult or impossible to do in public matches.
Original Game Settings
In Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2, settings such as game type, score limit, and time limit could be set, as well as Hardcore Mode. Players could also set health, respawn options, Headshots Only Mode, and, in Call of Duty 4: Modern Warfare, Old School Mode. These options can all be edited for split screen as well. Players earn no XP from private matches. However, there was a glitch which gave the player XP in Private match via hacking.
Call of Duty: Black Ops Settings
In Call of Duty: Black Ops there is a much greater level of customization. The lobby host is able to customize the effects of perks, and can also assign default classes that must be used. All weapons, perks and equipment are unlocked for use in the custom classes. Killstreaks are also customizable by the host. Customized gametypes can be saved and placed into a players' File Share. Like private matches in previous Call of Duty titles, no XP or COD Points are gained or lost from playing in a private match. Up to 18 players may participate in a single Private Match. Players also have the ability to save custom gamemodes into the community, where other players can browse and select matches that deem interesting to them. Also, certain community game modes allow the player to play Wager Match Private matches, which is normally not possible.
Call of Duty: Modern Warfare 3 Settings
Private Matches return in Call of Duty: Modern Warfare 3. It offers even more customization than in Call of Duty: Black Ops, with various custom gamemodes such as Gun game and Infected , and allows one to Rank up and Prestige.