Orc chains are a form of ladder that are used by evil factions in the mod. They are simple chains of Orc Steel, which can be used as decorative blocks or as makeshift ladders. Like rope, chains don't have to be placed on a block, as ladders do, and instead merely have to connect to a block at the top of the chain. They can only be placed on the bottom face of a block, however, unlike rope. Chains drop themselves when broken, and when a chain is broken all chains connected below it will break as well.
Orc Chains fit in very well in Orc builds, aesthetically speaking. In terms of function, they were added as a sort of counterpart to Elven Rope.