Erotic sensation play is a class of activities meant to impart physical sensations upon a partner, as opposed to mental forms of erotic play such as power exchange or sexual roleplaying.
Sensation play can be sensual, where the sensations are generally pleasing and light. Many couples that would not consider themselves active in BDSM are familiar with this kind of play: the use of silk scarves, feathers, ice, massage oils, and other similar implements.
Sensation play in BDSM can also involve sadomasochistic play, involving the application of carefully controlled stimuli to the human body so that it reacts as if it were actually hurt. While this can involve the infliction of actual pain, it is usually done in order to release pleasurable endorphins, creating a sensation somewhat like runner's high or the afterglow of orgasm.
In BDSM play, the top (or dominant) introduces and controls the sensation to the bottom (or submissive).
The sensation can come from just about anywhere, provided that the implement and its use fall within the negotiated terms of the interaction or relationship. Sensation play is limited only by one's own imagination and a lot of variation can occur from one play scene to the next. For example, chains could be left in a freezer for some time and then laid across the body of a bottom or submissive. Or the chain could just be brought out of a bag and run across the body or perhaps just rattled loudly to invoke a response.
Note that safety and consent are of paramount importance in the practice of more intense forms of sensation play. People engaging in such activities should adhere to the principles of "safe, sane and consensual" or of "risk-aware consensual kink".