Single player may be what brings you to the game but multiplayer will keep you playing the game.
The single player campaign is easy but it is fun.There are also the secret death cards to look for so if you got some spare time try looking for them.
The multiplayer mode is awesome, there are alot of different game types to choose from and many things to unlock like weapons, and perks. However the spawn system is bad at times. There is also a co-op campaign mode and you can activate those death cards you found in single player and use them to make the game easyer or harder depending on the card.
Nazi zombies is probably the best part of world at war, runinng around building baricades to keep the zombies out and connecting teleports etc.The mode can have up to 4 people play togeth and it's alot more fun with other people. This mode will go on and on untill everyone dies and after playing for a while it can get boring.
A great game over all and if your a fan of first-person shooters and haven't tryed the game out you should definatly do that.