SharePoint 2013 adds the concept of Pinning to taxonomy and the term store management tool. You might first encounter the option to Pin instead of Reuse on the page to add a catalog connection from cross site publishing or as a new node in the menu for a term in the term store management tool titled "Pin Term With Children." While at least one reason for choosing Pinning is better understood in the context of search driven publishing the tangible difference between the two is actually concise.

The difference is that while Pinning maintains a certain relationship (shared properties) between source term and Reuse instance on a one on one basis as Reuse does, Pinning also updates the hierarchy regarding the removal and addition of terms anywhere within that hierarchy.

Not to be mistakenly assumed with “hierarchy”, Pinning does not include propagating the order of terms in the hierarchy as custom sort would still be considered a local property of the term. You can think of Pinning as a special form of Reuse as you are still conceptually reusing the terms and most rules of Reuse continue to apply.