A collar can be anything from ring, anklet, to necklace. There are dress collars, work collars, play collars. I personally believe that a collared sub should have several so that she can be outside with family, friends or business people without having to explain why she is wearing a dog collar.
I have 2 for minx at this time and more will be on the way. It isn't to advertise who we are that she wears one it is so that she knows who owns her.