If I mark something as spam in Gmail, does that modify only my spam filter or does it affect the spam filter for other users?
Gmail users play an important role in keeping spammy messages out of millions of inboxes. When the Gmail community votes with their clicks to report a particular email as spam, our system quickly learns to start blocking similar messages. The more spam the community marks, the smarter our system becomes.
I'll complement the other answer and say that spams now have an explanation for why they're there. For example, I see sometimes at the top:
Why is this message in Spam? You previously marked messages from firstname.lastname@example.org as spam. Learn more
Which links to:
What you need to know:
After you report spam for several messages from the same sender, our system will learn from your behavior and might continue placing that sender's messages to Spam even if you don't specifically report them.