In his often-cited book, Notes from the Underground: Zines and the Politics of Alternative Culture, Stephen Duncombe defines zines as Noncommercial, nonprofessional, small circulation, self-published magazines that are not created to turn a profit. They are associated with counter cultures and marginalized peoples, handmade - often with a DIY or collage aesthetic - and use both text and image to make meaning. Both text and images can either be original, as in created by the author or artist, or appropriated from another source, as in the case of collage.
(Portland, OR: Microcosm Publishing, 2017).
This timeline by Duke University gives an excellent (and brief!) history of zines.
Many of these resources are available to download and print as well as read online.