A zine is a very specific style of printed booklet that is usually but not exclusively, about a niche subject. The graphic style of a zine is usually quite homespun, perhaps even a little low res and gritty. You would often expect to see imagery that appears to be cut out and stuck in place by hand with typography that also looks a tad borrowed from elsewhere.
Zines often have a limited colour palette but both muted and vibrant hues can be used to form something different than a standard booklet full of carefully placed imagery and neat text.
The style is only cohesive across the genre because it is meant to look like it has been hand made and hand-bound in a hurry to pass out with urgency. If the style of a zine is really very different from what you normally produce, the result will be more impactful. It could even be seen as a form of guerrilla marketing in this context.
When you think of zines, think of hand collated, stapled booklets that hold important and interesting information about something slightly obscure considered niche. Back in the day it would have been the publication of choice for underground communities. The Rebel Alliance would most certainly find this style of publication useful against the oppression of Palpatine and his Sith Lords.
Today, however, zines are mostly designed for booklets that break the traditional rules of imagery and typography. They stand out and are in stark contrast to a conventional brochure or product booklet. Their somewhat chaotic, colourful, and charming appearance allows for brands and businesses to have fun with print and to push the boundaries of marketing.