Here's a video Tim Flagler produced on how tippet rings are used to attach tippet to the end of your leader. I've never used them, but some of the anglers I fish with do use them, and like them for certain applications. I know others that think they are not worth the effort, or they think using them is akin to spin fishing. Personally I don't think they are a good or bad thing, they are just another way to rig your leader/tippet, and they may work for some and not for others. I do plan to try them at some point for no other reason than to form a first hand opinion on them.
Sharpen your hooks!