Collaring is a common practice in the BDSM community. People in all kinds of Dom/sub relationships often like to have something physical to symbolise the bond between them, be that a Daddy and his little girl or a Dominatrix and her slaveboy -and the giving and wearing of collars is frequently used in this way. For many people, a collar is a clear sign of a deep commitment and has a sincere emotional meaning for them. Of course, the objects used in this way aren't always literally collars--some kinky couples like to symbolise their ownership using bracelets, anklets, rings, chains or any other adornment that has a particular meaning for them.
There's more than one 'official' definition of what the letters in BDSM stand for, and plenty of people don't agree on their exact usage! However you use the term, though, it's a catch-all way of describing a whole bunch of kinks and fetishes that are more common than many people imagine: bondage and impact play, sadism and masochism, domination and submission.
Safety, consent and respect for boundaries are all key in the practice of BDSM, and it's vital to negotiate properly with your partners before getting into anything heavy-handed. Thankfully it's not so hard to learn - most places around the world have their own BDSM communities that are only too happy to take in newbies and help them learn the ropes.
For some, it's important to find ways to represent their social and emotional relationships as an inherent part of their physical form. This can be a key part in a BDSM dynamic, and for a certain kind of submissive - usually one who is highly masochistic - there's an irresistible appeal to being branded. Branding a human is much like branding an animal: a piece of sterilised metal is held in a fire till it's glowing hot, and then placed quickly onto the skin to leave a permanent scar. Many of these scars are intricate and beautiful, just like tattoos. Our one word of caution is this: don't take branding lightly, and only ever experiment with it in the company of someone who has training, experience and preferably some kind of qualification. It can be highly dangerous, and it isn't something to go into without forethought and planning.
Image: Michael Hemmingsson. Flickr Creative Commons license.