There are different SharePoint objects that workflows can execute on. Like:
Workflows can run in SharePoint list items. You can directly attach the workflow to announcement list, task lists and issues lists etc.
We can directly attach a workflow by the Workflows menu from the ECB menu.
We can attach workflow to an infopath form. You can see some Infopath tutorials here.
Content type is an important concept of SharePoint. We can attach workflows to content types. We can also attach one or more workflows into a content type. When a workflow is deployed into a content type, new instances of that workflow
can be initiated wherever list items of that content type exist. You can also see this article on How to attach a workflow to a content type?
Now we can bind a workflow to a SharePoint site, which is then called a site workflow.
A workflow deployed onto a site can run actions on and react to events across all lists, document libraries, and items in that site.
A workflow that would be well suited for running at the site level could check each document within that site and ensure that all the documents have been routed for approval and that none have been declined.
Workflows can execute across an entire SharePoint site and are initiated from within View Site Content.