The round stitch ring markers come in small (10 count) and large (20 count) sizes.  The rings are inserted on the knitting needle to mark as a guide for increasing and decreasing a stitch.  Also, for marking at the beginning and ending of the round.  Come in colors of purple and lime.
 
For needle sizes
Small (sizes 0-5 needles)
Large (sizes 5-11 needles)