KUDOS
Kudos are a feature of the Archive, and a quick and easy way to let an author know you've liked their work. You can only leave kudos once on a fic if you're logged in to your account, even if you have multiple pseuds. Also, you can't leave kudos on every chapter in a fic, but you can leave kudos on individual works in a series, as they are separate works.
Authors generally love it when people leave kudos. We love it even more when people leave kudos and comment on the fic/chapter. Some authors state somewhere in the author's notes or summary or profile if they would like you to leave a comment in addition to kudos.
COMMENTS
Comments on a fic are an author's lifeblood. We love those things, and seriously, write an essay for all we care (max character limit is 10,000). Comment chapter-by-chapter on a multi-chapter fic if you want. You do not have to have an account to leave kudos or comments, but authors can regulate the comments section for their individual fics so that either guests and registered users can review, only registered users can review, or all comments must be viewed by the author before being published so that the general public can read them. (Similarly, a work can be set so that only registered users can view it.)
This is where etiquette comes in. The comments section on AO3 is set up to mimic threads and discussion on LiveJournal circa 2001 - 2008. Fic authors can delete comments/comment threads and even lock threads if necessary.
Guest comments can be marked as "Spam" by the author and reported to the Abuse team for an investigation.
As a default, comments are hidden beneath the work's Kudos list, so you have to click on the "Comments" button at either the top or bottom of the fic page to open and view them. You can also hide comments by clicking on that same button (which will change to "Hide Comments").
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A Wattpad User's Guide to AO3
RandomTutorial/Guide for Wattpad users who are migrating to AO3, have never used it before, and/or are unfamiliar with the culture/etiquette on AO3. Written by a Fandom Old who has been using the Archive of Our Own since 2011.