What's a tippet ring? In short, it's a tiny metal ring with a simplistic purpose: to join the leader to the tippet. You tie your leader to one side of the ring, your tippet to the other side, and viola, you have instantly created a connection. You may ask "Why not just connect the two sections together with a single knot like I've always done?" That's a good question with a simple answer.
A big advantage of a tippet ring is that it will save your main leader from getting shortened. Whether using a knotted or knot-less leader, over time your tippet section will get shorter and shorter as you change flies or cut off frayed sections. Before too long, it's largely gone and you'll be forced to tie on a whole new tippet portion. The problem is, each time you tie on a new tippet portion the main leader section gradually gets shorter since some of it must be used to tie the connecting knot. This is where the beauty of the tippet ring comes into play. You can now just cut off the old tippet piece and tie a new tippet section to the ring using a quick, simple knot (another advantage!) like an improved clinch while leaving the main leader untouched. Tippet rings are awesome for fishing droppers too since you can tie your dropper section directly to the ring. Want to fish a dry fly? No worries. RIO states they are light enough to be used with dries, and when I tested the "trout" size ring along with a tiny Elk Hair Caddis (the smallest dry in my box) the fly floated readily.
RIO tippet rings come loaded onto a swivel for easy storage and are available in two sizes: Trout which are 2mm in diameter rated at 25lbs, and Steelhead which are 3mm in diameter rated at 45lbs. They are constructed from a steel alloy and are corrosion resistant, but should be rinsed off after any saltwater usage. Price:$6.95 (pack of 10) for either size.