Many drapery styles, like pinch-pleat, require rings to attach your curtain to your drapery pole. The small circle at the bottom of the rings is called an eyelet. Drapery pins attached to your curtain fit through the eyelet and connect your curtain to the ring. For flat panel treatments, alligator clips can attach to the eyelet as well. It's a good idea to forego alligator clips and sew your rings directly to your panel if your drapes are long or heavy. Clips are easy to use, but they don't do a very good job holding full length panels or lined or interline drapes.
Rings for curtain rods are always named by the pole diameter they fit. When you see a ring called "Two Inch Ring", it doesn't mean the ring itself is only two inches, it means the ring is made to fit a two inch diameter drapery rod. When selecting a ring, if you already have a drapery pole, you'll need to measure its diameter first. Typically, if you need rings for an existing drapery hardware treatment, it's best to replace all rings instead of just a few. Unless you know the exact brand, collection and finish of your existing curtain rod, the new rings will really "stick out". Finding good replacement rings for an existing treatment is a very difficult task.
Some collections offer different ring styles to add interest to your treatment. You may find some wood style poles with metal rings available. It's becoming quite popular to intermingle different materials within your drapery hardware treatment to provide contrast.