These days they tend to be single sheets of paper folded to look like a small 8 page book. That's the modern cute version. "Back in the day" - meaning the 70's and 80's a 'zine was a mimeographed or photocopied hand made full sized magazine, typically on what we'd call cheap printer paper today.
They were punk rock expressions, with large amounts of collage art, mix media art + poetry, and amateur fiction, and sometimes fan fiction. It was creative expression for people with no outlet: the punks, angry about economics. If you think punk was about music, that's the lie, it was about dead end careers.