megabytes

Downloading iOS upgrades/updates.

Downloading app updates.

Streaming video content.

Having apps and Web browser website tabs constantly updating data in the background.

Always automatically fetching emails

FaceTime calls

Text messaging.

You are using bandwidth when you download something and, separately, apps, movies, etc. will add up whatever their size is when you download them. Everything has a size (an app may be 10 mb or 500 mb, each one is different) and, if you have a plan that limits you in downloads or data usage, you need to decide which app or update you want to download. An iOS update for instance can be up to 1 - 2 GB (there are 1,000 mb in 1 GB); and movies are usually quite large. I can't give an example since I've never downloaded one, but I'd imagine that a 2 hour movie is quite large.