CoD: WaW Zombie Nazi Pack is a set of 28 maps all created by different users as a give back to the community. All these 28 maps are created in the Nazi Zombie mode of the game that you can play after you complete the game atleast once. Some of these maps are Nazi Zombie lost island, Nazi Zombie Rathaus and Nazi Zombie Anzio Map etc. The complete list of all these maps and their download links can be found here at FilePlanet which has made a great efforts of bundling all the maps under a single map pack.
When you install this Call of Duty: World at War Zombie Nazi Pack, after you complete the actual game, special Nacht der Untoten (Night of the Dead) achievement is unlocked where you can play Nazi Zombie mini games in co-op with your friends. Download CoD: WaW Zombie Nazi Pack from the above link.