A list workflow is the type of workflow that was available in SharePoint 2007. Because it has the context of the list for which it was created, list workflows automatically have access to the values of the custom fields for the list item on which they will run, such as the custom Notes field for a document library. List workflows cannot be made available to other lists or libraries on this or other sites. To have the same workflow functionality for multiple lists you must manually recreate the workflows in all locations.
If you know that you will only need the workflows you are designing for a specific list, the list workflow has the advantage of automatically making available the custom fields of the lists.
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.
You can also export a reusable workflow from one site and then upload and activate that workflow in a different site. For example, you can create a reusable workflow in a test environment, test it, and then export it to a production environment. SharePoint Designer 2010 supports exporting a workflow as a template.
Reusable workflows, by default, don’t have the context of a specific list or library. Therefore, by default, they provide only the columns that are common across lists and libraries, such as Created and Created By.
If your reusable workflow requires certain columns to be present in the list or library that you associated it to, you can add those columns as association columns. Association columns get added automatically to a list or library when a reusable workflow is associated to that list or library.
When you create a reusable workflow, you can alternatively choose to filter your reusable workflow to a specific content type. This enables you to work with the fields of the content type in SharePoint Designer 2010. For example, if a reusable list workflows is associated with the Document content type, you view and use in your workflow fields that are specific to the content type, such as Document ID. Then, in the browser, you can associate your reusable workflow either to a specific content type or to any content type that inherits from that content type. If you associate a workflow to a site content type, you make that workflow available for all items of that content type in every list and library on the site to which that content type has been added. You can even make it available for sites in a collection if the workflow is configured to be a Globally Reusable Workflow.
If you want users to be able to use the workflows you are designing on multiple sites, lists, libraries, and content types, a reusable workflow will probably best meets your needs. We expect that most workflows for SharePoint 2010 will use reusable 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.
In the browser, you start a site workflow or view the status of running site workflows by clicking the Site Actionsmenu, click View All Site Content, and then clicking Site Workflows.
If you want to create a workflow, but don’t need a list, library, or content type for the workflows, a site workflow will probably best meet your needs. For example, you can create a site workflow as a way for people to provide feedback about your site.
Site workflows can only be started manually.
(Copied from https://support.office.com/en-ca/article/Introduction-to-designing-and-customizing-workflows-32c9c0bf-5e20-4f74-8b9c-d3ea79f2962b)