What adblockers block
The thing about adblockers is that they don't really block ads; determining what is and isn't an ad is an AI problem and we're nowhere near solving those. So what adblockers really block is signs and patterns that designate or suggest ads. The primary patterns that adblockers can use are URLs of resources being requested (such as images and other additional content) and the surrounding HTML context of these requests (including things like CSS tags).
(Many adblockers will allow you to inspect the patterns that they use in their preferences or advanced configuration system.)
No set of heuristics and patterns can possibly be complete. So in practice adblockers only block major sources of ads, because these are the sources of ads where the work of writing rules really pays off in a reduction of ads. In other words, adblockers mostly block decent-sized ad networks and pervasive places with ads like Facebook, Google, and YouTube.
Unless someone really goes out of their way to write rules, adblockers
do not and will never block hand-crafted ads on small sites; there
are no non-AI heuristics that can reliably figure out which bits
of Jane's Fishing Information are ads and which bits aren't. Similarly,
adblockers mostly don't block the various small scale ad networks
that are active in niche areas
like online webcomics, ultimately
because they haven't annoyed anyone enough to write (and update)
the blocking rules necessary.
The direct corollary of this is that even pervasive use of adblockers cannot kill advertising on the web. The only thing they can kill is mindless, computer-targeted advertising at mass scale. Such large scale advertising is attractive to a lot of people for a lot of reasons, but it is not the only advertising model for sustaining modest websites through ads.
(Adblockers can kill advertising on large sites because even if the large sites do entirely custom advertising systems, they are large enough that people will find it worthwhile to write the rules necessary.)
PPS: As it happens, large scale advertising networks are already often not an attractive model for modest websites with dedicated audiences because of various fundamental drawbacks in the model (like lack of control over what ads show on your website).