B: Bondage
Bondage: The act of restricting or restraining a partner.
D: Discipline / Dominance
Discipline: The act of setting rules with punishments for breaking them.
Dominance: The act of taking control or holding the power in a sexual scene or fantasy.
S: Submission / Sadism
Submission: The act of yielding control or giving up the power in a sexual scene or fantasy.
Sadism: The act of inflicting pain, humiliation, or intense sensation for erotic pleasure.
M: Masochism
Masochism: The act of enjoying pain, humiliation, or intense sensation for erotic pleasure.
Read more here - Babe Blog: How to start exploring BDSM