I actually have a timely example from the sub today!

At this link, you can see my response to a (currently-removed) comment which included an Amazon affiliate/referral/whatever you want to call them link/tag.
The comment has a link to this compressor — http://www.amazon.com/dp/B005Z8HAUK — but instead of the link ending in the Amazon Standard Identification Number code for the product (B005Z8HAUK), it ends in a referral tag (B005Z8HAUK?tag=[AssociateID]).
The part including and after "?tag=" are the "affiliate" part of the link; Amazon Affiliates receive money for every product purchased from following that link (and might receive payment just because someone clicked it, I'm not sure), which is a violation of Reddit's spam policy, and the comment was auto-removed by the spam filter. I manually approved it conditional on the commenter's editing of the comment, mainly because the comment itself still contributed to the discussion, but since they have not done so yet, I pulled it again.
There are also bit.ly shortlinks for Amazon referral codes (e.g. amzn.to/A1b2C3d), but shortlinks are also auto-removed by Reddit for their potential for malicious content, so you don't see them very often.

TL;DR: if it includes "?tag=" then it's a referral link.