They are worn on the ribbon bar. For example if you see three bronze stars:

• They are "ribbon devices" for participation in major campaigns
• The three stars on an ETO ribbon means a veteran served in 3 campaigns in the European Theater of Operations. If a vet was in more than 5 campaigns - the bronze stars would've been replaced by one silver star.

That's what I thought. So why do all the medals on Ebay have them on both the medal and ribbon bar? I knew I'd get the right answer here. Thanks again.

I should have made that more clear. The Bronze Campaign Stars can be worn on the ribbon of the actual medal too. See this example.