For a workflow to be useable on a SharePoint site, it must first be associated with a list, a library, or a particular content type. As a Web designer, you have to make this choice based on your requirements. After the workflow is associated with a list or a library, an instance of it can be started by firing it manually or setting it to automatically start whenever an item is created or changed.
Only a single instance of a workflow can be run on a list item at one time. This means that while an instance of a workflow is running on an item, you can't start another instance of the same workflow on the same item. You can, however, start an instance of another workflow on the item. It's recommended that you don't run more than 15 workflow instances on the same item at one time.
You can associate a workflow with the following SharePoint assets:
Associate with Lists: When you associate a workflow with a list, instances of the workflow can be started for the items in the list. The item on which the workflow is running is called the Current Item in the workflow context.
Associate with Libraries: After the workflow is associated with a library, its instances can be run on the document, pictures, etc., that belong to the library.
Associate with Content Type: By allowing you to associate a workflow with a particular content type, SharePoint lets you use a single workflow across multiple lists and libraries that use that content ...