Reusable List Workflows -
You can create a reusable list workflow (reusable workflow) in the top-level site in the site collection, and that workflow can be made globally reusable - meaning that the workflow can be associated to any list, library, or content type in the site collection. You can also create a reusable workflow in any subsite in the site collection; this workflow is available for reuse in that particular subsite.
Site workflows - A site workflow is associated to a site - not to a list, library, or content type. So unlike most workflows, a site workflow is not running on a specific list item. Because of this, many of the actions that are available for items not available for site workflows.