The workflow designer interface is where you define the complete logic of the workflow. With the Workflow Designer, you can attach a sequence of conditions and actions to a list or library. SharePoint Designer provides reusable workflows that can be used multiple times within the same site.
There are 3 types of workflows that can be created using SharePoint Designer: List, Reusable, and Site. Following is a quick breakdown of how the 3 SharePoint Designer workflows are used and why you would use them:
List Workflow - Using this mechanism, you attach the workflow directly to a list or library on the site. Use this workflow when you are making a workflow that's very specific to a list or library and does not need to be later used on a different list or library.
Reusable Workflow - This type of workflow is created with reusability in mind. Create a reusable workflow when you intend to attach it to a content type and use that content type in a list or library.
Site Workflow - Site based workflow does not require to be attached to a list or library. It works on the site itself. Use this workflow if you do not want to restrict the automated process to a list or library on the site