Shackles are used to connect pieces of rigging to connection points (on vehicles or static items) or other pieces of rigging, and there are two major shackle types used in vehicle recovery: Hard [Metal] Shackles and Soft [Synthetic Line] Shackles.  

METAL SHACKLES: These come in different configurations, with one of the most common configurations used in vehicle recovery being the Screw Pin Bow Shackle.  Metal shackles are heavy, exhibit the toughness associated with whatever metal or alloy they are made from, and are typically fixed to vehicle connection points.  Metal Shackles are well-suited for use on connection points that have sharp edges or are otherwise not smooth.  

SOFT SHACKLES: These are relatively new, and take advantage of the unique properties of HMPE, a very strong and lightweight synthetic line.  Because Soft Shackles are soft, they will not damage the surface of whatever they are attached to, and their light weight makes them far safer than Metal Shackles if there is every a line failure - metal objects are much more of a missile hazard than a lightweight piece of line.  Soft Shackles are great for connecting line to other line, or for use on connection points that do not have sharp edges that lead to cutting.