They are real plants. They're a type of algae, not really moss. (They're fuzzy, not slimy, too.) The currents in their natural habitat are what cause them to grow in a sphere. They're pretty hardy, very difficult to kill, and they grow really slowly.
I've heard they help stop other algae from growing on the tank walls. Some fish will eat them, but betta fish like laying on them!
You need to take it out of the cup it comes in, gently squeeze the water out, then rinse it under a gentle stream of water, then plop it in to your tank.
Every once in awhile they need to be rolled over in the tank so they don't get brown on the side they sit on. About every one to two weeks you need to take it out, gently squeeze the water out of it, roll it in your hands to help keep it in shape, then run it under some water.
EDIT: Some places try to pass off java moss wrapped around a weight and tied to it as a moss ball, but to my knowledge, Petsmart's are legit.
Last edited by miscanon; 07-11-2013 at 12:09 AM.
Reason: forgot some info