"To suffocate" means "to die from lack of air" or "to kill someone by depriving them of air." The method is not specified. "The avalanche victims were buried deeply in snow and suffocated." "Halon is odorless, and can suffocate you without your being aware of what is happening." "To asphyxiate" is a synonym for "suffocate." It is more formal and more technical. In ordinary speech we might say "he suffocated;" a medical examiner's report might say "he died of asphyxia." "To strangle" is squeeze someone's throat, and thus close their airway. "To throttle" is a synonym for "to strangle." "To smother" usually means being covered by a huge mass of something soft. It doesn't necessarily mean suffocation. One traditional U.S. dish is "liver smothered in onions," meaning that the cooked piece of liver is covered by soft, cooked onions. If a grandmother picks up a baby grandson and kisses him quickly, over and over again, we say "she smothered him in kisses." In the case of suffocation, it suggests something soft but solid--a pillow, a thin plastic bag, deep snow--covering the face. Also, "to choke" is to close off the airway, entirely or partly, either by strangling from outside, or from something getting into the trachea and blocking it from the inside.

Strangle and throttle involve using something such as your hands or a rope to physically compress the airways. Smother involves physically preventing somebody from breathing by pushing something down on top of their face e.g a pillow. Suffocate and asphyxiate mean exactly the same thing - they mean that a person's air supply is cut off. Strangling/throttling and smothering are different ways to suffocate somebody.