A voting ring is a group of people that all upvote each other's posts. But most of the time, it lacks that level of organization -- usually, ring-voting just refers to having all your friends upvote your post for you.

One really easy way to trip HN's voting ring detector is to send your voters a direct link. However, in my experience, this acts less to detect voting rings than to weed out HN newbies. All my friends just post on facebook with something along the lines of "go search the New page for <x> and upvote, please!"