Which of the Windows updates are really necessary?

I downloaded a lot of Windows updates in the automatic mode.

Are they all necessary? Can I remove some of them?

How do I know which ones are necessary

A lot of people probably wonder this: do I really need to download all 50 updates? My precious bandwidth! Ranon's answer goes into some good detail about the different types of updates and how important they are. A blog post about this might optionally talk a bit about WSUS for managing updates, and various update-packaging software (do those still exist?) so you don't have to re-download on every machine.

I won't have time to write about this in detail, but if it wins and nobody else picks it up, I'll post a quick summary to the blog.