It's a little round acrylic strap button, which has adhesive on the back to stick to your phone. Once it's firmly secured to your phone, you can get your cell phone looking stylish by threading through the phone charm of your choice. Some people even put more than one on and hang multiple charms on their phones.
The button is approximately 9/16" in dismeter (about 14mm) and is about 7/32" high (about 5.5mm).
Check it out at cellchic.etsy.com!