While every weapon has training to build power and strength, the rings are often considered just a training aid by many styles. They do many of the basic drills and techniques but tell people that is only for physical training. Maybe that is all they know. Some of the drills involve wearing the rings as bracelets and going through the forms. This builds the forearm conditioning and joint strength. The extra weight randomly pulling and pushing on the forearms helps build spinal strength and helps build the skills which help defend against wrist grabs. Many styles use the rings as an aid to help develop power and control in small circular motions of the wrist, elbow, and shoulder. This has an obvious benefit to other weapon training and is early training for inch power development. Not the inch punch from bruce lee, but inch power which is different and has more use. The smaller rings are also used to help develop different types of isometric claw strength with some application as armor in combat application.
As Styles go the smaller rings are used by Hung styles, Wing Chun, and other ShaoLin styles which have a red fist influence. The larger rings are used in pairs by styles with a longer range and circular motions. The styles wich use rings are not rare, the desire to train rings is rare. The largest rings are used by monkey fist. monkey uses on large ring which is big enough to fit the body through. The acrobatics lean toward this size ring and the monkey stylist will often use the ring to tie up the opponent(s) and then the ring provides a convenient handle to throw them.
Other variations on the concept are wheels and hoops. Hoops tend to be thin and aren't used much. They are very delicat but some styles train with them.
Wheels are the most commonly recognized of these. Wheels are flat and a closed circle. They tend to have a section designated as a handle, usually wrapped with cloth, and can be sharp or dull depending on the style/person. If sharp, the wheel will have an arc of metal over the handle to protect the fingers. When blades are attatched to the wheel it falls under the name of Wind Fire wheels. Wind Fire Wheels tend to be used with internal training and by internal styles. The Wind Fire Wheels will have anywhere from one to nine attatchements, always an odd number. The handle may be covered by an arc or a crescent blade. The wheels are always used as a paired weapon. In training, they are used like rings in that they are held and kind of swung at people. Internal styles try and manifest the energies of the wheels but their techniques are still similiar. A wheel form looks like somebody just held the wheels while doing a hand set. The external styles often use a basic punching form with minor variations for different blade designs. Their apparently isn't any difference in use for the number of blades either. Someone will have to work on that.