A workflow can be categorized with a Process Category. The primary usage of this categorization is on the Workflow Launchpad, where the available workflows will be grouped together by category.
Process categories are customizable by organization, to update the category:
From the Administration module and click on the Workflows page
Click "View Published Version" for the workflow you want to update
Click "Edit" in the top right corner
Open the Picklist for "Category" and switch to the category you want for the workflow
Hit "Save Changes" and the workflow will now be in the updated category