Shime-Waza (Choking Techniques) is one of the groups of Katamewaza (Grappling Techniques)
The execution of choke is in practice a subtle art because of the potential danger for the opponent. There are three fundamental manners for chokes: compression of the neck-veins which restricts the flow of blood and oxygen to the brain, compression of the trachea,and compression of the chest and the lungs which prevents breathing.
Pictures of Choking Techniques
Also check out Pinning Techniques (Osaewaza) and Joint Locks (Kansetsuwaza).
Or vist animation Throwing Techniques (Nagewaza) page.