Spring ring clasps have a tiny spring inside, springing them back closed after you open them. Small spring rings are popular for bracelets and choker necklaces. Larger spring rings are sometimes used as charm holders, allowing you to clasp your charm onto just about anything: long necklace chain, shoelaces or backbacks. People normally pair spring rings with a medium-large jump ring as the other half of the clasp.
Cheap spring rings have a tendency to be plated shut, and you may resort to using small pliers to open them the first time (breaking through the plating), and then they usually work fine after that. Superior quality spring rings open nicely the first time (and every time after that) and promptly and securely spring shut.