Watch this video. It shows you some differences with the WaW filter on and off:
YouTube - Call of Duty 5 World at War - How To Filter Graphic Content
The filter really does change a lot, though. The cutscenes that are violent get pixelated and there is MUCH less blood. In fact, none at all. No gore either. Ask your mom/dad about Nazi Zombies. That one is really bloody and gorey. My parents don't mind me playing that cause it's not real people. They care about me killing real people only (ex. multiplayer and campaign). That's why I play in my own room. I never show them gameplay. Here's a good question, where do you play your PS3?