Binding is a physical, exotic advantage, described in Basic Set, p. 40. It has the special enhancements Engulfing, Only Damaged By X, Sticky, and Unbreakable, and the special limitations Environmental and One-Shot.
It is referenced in GURPS Powers™, p. 9, on choosing a power, pp. 42-43, on the form that Binding may take, and p. 161, on using Knot-Tying with Binding when not in combat. P. 43 also gives the new special enhancements Constricting and Suffocating.
Page 42 says it can have the Wall enhancement.
http://www.sjgames.com/gurps/faq/FAQ4-3.html#SS3.2.32 emphasies it grapples.