A collar resembles a popular accessoire in Second Life®.
It can be a "simple" piece of jewelery, often worn by neko or furry avatars, though many collars are scripted in order to allow the wearer certain features. These are especially popular within the role play community of Second Life. Quite common features let the wearer grant certain permissions to a third person, like animating the own avatar, limiting it's mobility, tracking if the collar is worn or not, etc.
These kinds of collars are sometimes equipped with an API to allow interaction with other devices.
Besides several commercial closed source releases, there is also a popular set of open source scripts available for content creators. The so called OpenCollar scripts are provided and documented at Google Code. Also an open LSL Protocol, called LockMeister System, is available for developement in order to enhance compatibility to other products.
It is not clear if or which of these scripted collars are counted amongst adult content, which would only allow them to be sold at regions having an adult region rating. However, it is clear that they aren't allowed to be advertised with adult content keywords, when they're supposed to be sold on PG or Mature regions.