BDSM (Bondage & Discipline, Dominance & Submission, Sadism & Masochism) is an umbrella term for sexual practices which play with power relations, pain and personal limits. This requires a good foundation of trust between all involved parties. In BDSM there are simple tools, such as, safeword and consent. [1] [2]

A safeword is an emergency brake in every BDSM interaction. Mostly, the traffic light system is used:
Green: "Carry on"
Yellow: "Take it easy"
Red: "Stop, Now!"
To disregard a safeword is one of the worst offences because it violates the safe context where interaction takes place. [2]

Consent refers to the mutual agreement before sexual actions. The topics are desires, needs and hard limits provided that these agreements can change any time and are not absolute. A consent doesn't relieve an individual of the duty to always be attentive and mindful with his/her partner and notice possible changes in the other's attitude. [1] [3]

Sexual interactions are often based on the assumption that the other person has the same or similar ideas of sexuality. This is what may cause boundary crossings which can result in traumata. This situation can worsen when one of the participants is drunk or intoxicated. [2] [3]

Reaching a consent and an attentive interpersonal contact is not only important in the BDSM context but also within the limits of "normal" sexuality because it's proven that there's one factor which improves any kind of sexuality: communication. [2] [4]

Further Links:
Tea Consent, Emmeline May and Blue Seat Studios, Copyright ©2015 (YouTube)
ROCKSTAR-DINOSAUR-PIRATE-PRINCESS, Emmeline May (Blog)
TOP 10 BDSM MOVIES, Bondage Network, 2014 (YouTube)