Trying to explain what an APA is is harder than trying to explain what a zine is, so the best way to understand is by ordering and looking at one, better yet 2 or 3 consecutive issues. So if you send 2, 4 or $6 to Idy, he’ll send you one or more samples—in old APA jargon those would be “spec” copies. The next one’s due out in February and I’ll have 8 pages in it.
Wikipedia has an article on Amateur Press Associations. It’s dry and undescriptive, so not very helpful, but it might answer a few questions for the uninitiated.