The BDSM collar is a symbol of being owned. Collars are often described as the BDSM analogue of a wedding ring. Certainly, a collar can indicate a strong level of commitment and care, but it doesn't have to. Just like the concept of slavery, ownership means different things to different people.
Sometimes an owner will hold a formal collaring ceremony, with friends and family as witnesses. More often it's private. And once in awhile a collaring is completely virtual. These collaring rituals can be romantic, crude or beyond gross, just like the people involved. Sometimes Master and slave will just spontaneously go off collar shopping.
A catty term that's used frequently by BDSM practitioners is the “Velcro collar”. A Velcro collar snaps off and on quickly. It's a contemptuous phrase describing a collar for folks who don't take the whole relationship thing seriously, and hop from one interaction to the another. A Velcro collar is quite different from a "play collar", which is a fun toy for Master/slave roleplay.
Another sort of collar is the "collar of consideration". This jewelry marks out ownership territory without any ownership commitment. The collar is supposed to scare off other suitors. It has nothing in common with an engagement ring. It's not a promise. It's a plan B, while the potential Master looks for a better slave. I don't respect people who resort to (or consent to) such nonsense.
An acquaintance of mine was looking to be owned, and was speaking to a potential Master in another city. He wanted to temporarily mark her as his, just in case she turned out to be good enough. He ordered her to wear a "collar of consideration" before she went to a BDSM event. And he even demanded that she get her own! She made her collar out of twine and ribbon. She was mighty proud of it. But I thought the guy was rotten and lazy. Needless to say, that relationship didn't progress very far.
A Master who wants the status of a collared slave but doesn't care enough about the symbolism to actually supply a collar, is telling the world loud and clear how well he'll treat her when she's well and truly owned.
The collar pictured above is available online from the Chain Gang.