It can be allmost anything from a ribbon around your wrist, to a clit ring with a bell, jinggle jinggle lol, (which happens to be mine..blushes) or even a tatto or a brand

alltough we have play colars and one that we use for more ritualistic acts of submision etc

it need not be a physical manifestation at all, it may only be a collar worn inside your heart, many of us our bound by our owners will long before a physical collar comes into the equation