Does the iso of the film dictate the iso in the camera?

So I'm new to film and just started using a Nikon F100. I've finally picked out what film I'm using but had a quick question before I start to shoot: Does the iso on the film have to be the same as in the camera?

I thought that whatever the iso on the film is, has to be the same iso as in the camera settings.

For example: whenever I shoot on 400 film, I set the camera to iso 400 so that they're the same.

However I've seen people say they've used 400 film but "metered at 3200."

Can I use 400 film but change to iso on the camera to whatever I please? If so, how does changing the iso on the camera affect the shot?

You can pull and push the ISO setting to change the metering, but you are still better off setting the ISO correctly and then using the visual queues from the camera metering. Normally if you pushed or pulled the ISO to a different level you would also be wanting to develop your own film so that you can adjust the processing to meet your goals. As you are new to film you don't really want to be worrying about this yet.

If you use ISO400 film but set the camera to ISO800 your images are going to come out under exposed, equivalent to halving the area of the aperture (1 full f/stop). Shooting at ISO 3200 would leave the film very dark indeed!

ISO is medium sensitivity. In a digital camera, the medium is the digital sensor. In a film camera, it's the film. ISO then to your question refers only to film. To accurately compute exposure, you must inform the camera about the ISO of the film. That's why you set the film's ISO on the camera.

There are times you want to push or pull the film for special developing purposes so if you know what you're doing, you can raise or lower the camera's ISO setting from the actual film ISO.