The same concept is also found in the spanko community.
The term usually implies that there is no deeper relationship beyond that of BDSM play. In particular, play partners are not necessarily in love with each other. A play partner can be outside one's range of potential love partners (e.g. in terms of gender, age, or looks). The criteria of who makes a good play partner may be different from the criteria for a lover or spouse.
There may or may not be emotional and/or physical closeness between play partners. There may or may not be sexual activities in addition to BDSM play. There may or may not be any role play. What most people agree should be there is a relationship of trust, responsibility and respect for the well-being, individual needs and limits of one's play partner. Due to the special and intimate nature of the fun shared, play partners will often become close friends after a surprisingly short time.
Play partnerships will often involve one top (or dominant) and one bottom (or submissive) partner, but the partners can also be switches. Switches may decide on one role with a specific partner and stick with it, but they can also change roles within the play partnership on mutual agreement if and when they feel like it.
How to find a play partner
Play and other partnerships
The concept of play partnerships is unique in BDSM and often difficult for "vanilla" people to understand. By traditional standards, one wouldn't do intimate or erotic things such as BDSM play, even when no sex is involved, with a person one is not in love with. The point is that BDSM play is also play in the sense of having fun and as such can well be enjoyed with a partner (who is reponsible, trustworthy and compatible) without the necessity of a sexual or love relationship with that same person. In fact, it's much easier to find a play partner than a real partner as the requirements will be much lower. Who cares if your play partner snores or squeezes the toothpaste from the wrong end, as long as he or she is fun to play spanking games with?
People may be explicitly looking for a play partner because they may not be interested in a closer relationship. Or they might already have a love partner that has no BDSM interest. Or they may be looking for a play partner to leave all options open — a play partnership can evolve into something deeper but doesn't need to, and if it doesn't, nobody loses face because the whole thing did not start with a love affair to begin with. Those who manage to find a long-term partnership that combines love, sex and BDSM in a way that leaves no major needs of either partner unfulfilled, usually find this the ideal case.