You can read the full set of FTC Guidelines here.
Let me start by saying if you are NOT a US resident or citizen, the guidelines don't apply to you. If you ARE a US resident or citizen, even if you are posting on a foreign website, they do apply to you.
Now, I am NOT a Lawyer (IANAL) You do not need to be a lawyer to make sense of the FTC Guidelines regarding affiliate links and product endorsements online. It's written in plain English with numerous examples.
Basically, what it comes down to is this - if there is a benefit for YOU, you must disclose such. If the affiliate links are in a post, a forum comment or an online article benefit you (such as the 5% you get back from OBS) you need to indicate such. Not the exact amount of the benefit, but that there IS a benefit for you. This applies to review copies too - if you are reviewing a product and you were comped, you must explicitly say so.
At The Tavern, every page has the same surrounding text, links, ads, lists. I prominently disclose exactly how much of a commission I get at OBS and OGS. These are at the top of the page, just under the header. This SHOULD cover me for disclosure of any affiliate links in my posts that announce products or are reviews, but IANAL. I could be wrong. It may be that I need more explicit disclosure. I am, however, acting in good faith upon my interpretation of the guidelines in question.
Now, if you do a review of a freely distributed product, or one you purchased for yourself, no disclosures are needed (but you would still need a disclosure for the affiliate links, if any.)
What about connections to products, publishers, authors and the like? I have started to fully (some have said TOO fully) disclose such relationships. I may add some humor to said disclosures, but that's simply how I am.
At this point, I know so many people and publishers that nearly every post mentioning a product will probably require a disclosure from me.
Do I think the FTC is going to come down on bloggers and the like in the OSR for lack of disclosure? Probably not. We are small fish in a small pool. Too much work for too little in return. Disclosure, however, keeps the hobby honest and that is a good thing.
Nothing in this post should be considered legal advice. The examples herein are how I am approaching these guidelines. Notice these are guidelines and not hard rules. As such, there may be gray areas.