Choking is also known as a foreign body airway obstruction. It's something physically in the way of air moving in and out of the trachea. In most cases, choking is caused by food. The foreign body may get stuck in the trachea, directly blocking airflow. Or, a large foreign body can also get stuck in the esophagus (where food passes on its way to the stomach) and squeeze the trachea from behind. Either way, the foreign body is inside the human body when it causes choking.
Choking can be treated if it is recognized in time. The treatment for choking depends on the age of the patient. Infant choking is treated differently than in adults and children older than a year. Choking in adults and kids older than 1 year is best treated with the Heimlich maneuver.
Strangulation is the restriction of airflow through the trachea via something squeezing from the outside. It's technically incorrect to say that someone was "choked" by another person. The correct terminology would be strangled. Whether caused intentionally or accidentally, anything wrapped around the neck hard enough to restrict airflow through the trachea is causing strangulation.
The treatment for strangulation is immediate removal of the device or object that's causing the injury. Strangulation can lead to severe injury and deformity of the trachea. It often requires advanced airway care that only a paramedic or doctor can provide.
Anyone who's been strangled -- accidentally or otherwise -- deserves a call to 911. An injury to the trachea may not appear to be serious right away, but swelling in the tissues around the trachea can lead to a secondary restriction of airflow a few minutes after the neck is free.