24/7 = 24 hours a day, 7 days a week. In other words, all the time. 24/7 D/s is a goal of many people in long-term, committed relationships, but everyone knows it's virtually impossible to stay dominant or submissive all the time.

B & D = Bondage & Discipline. Bondage is the practice of tying up or otherwise immobilizing a willing participant for pleasure or punishment. Discipline is the practice of the dominant partner instructing, correcting, or punishing the submissive partner. Both bondage and discipline can be physical, psychological, or sexual, or a combination of all three.

BDSM = Bondage-Discipline-Sadism-Masochism. This is a catchall acronym that covers a huge range of practices and preferences. It often brings to mind stereotypes of the black leather "whips 'n chains" variety, but in reality, most BDSM activities are extremely individualized.

CBT = Cock & Balls Torture (or Training). This is exactly what it sounds like, and is used on male submissives to inflict pain or humiliation by striking the genitals (by hand or with a flogger, paddle, etc.); enclosing them in a wide variety of rings, cages, and other devices; or attaching weights, clamps, or other objects.

D/s = Dominance/submission. It can refer to either the lifestyle or the participants. The dominant partner may be known as the Dom, Master, or Sir (if male); or Domme, Mistress, or Ma'am (if female); and the submissive partner may be known as the sub, slave, or pet.

OTK = Over the Knees. This is pertains to those who enjoy consensual spanking. 
 
RT = Real Time. Used to define face-to-face "real life" situations as opposed to online or phone. There are many who consider the "real world" as the only way D/s can be "real", but many who dominate or submit via modem or telephone will disagree strongly. Like anything else, it's all what you make of it.

S & M = Sadism & Masochism, or Sado-Masochism. Psychologically speaking, these are extreme mental conditions indicating severely unbalanced people, but in BDSM circles, S & M usually refers to the practices of consensual inflicting or receiving pain, and those who derive physical and/or sexual pleasure from doing so.