SharePoint workflows are pre-programmed mini-applications that streamline and automate a wide variety of business processes — from collecting signatures, feedback, or approvals for a plan or document, to tracking the current status of a routine procedure.
The following five workflows are pre-programmed (built-in) in SharePoint Online for Enterprises.
Customize workflows using SharePoint Designer
If you need more flexibility with a built-in workflow, you can customize it further with a tool like SharePoint Designer (SPD). You can also create your own original workflow from scratch. Using the Workflow Designer, you create rules that associate conditions and actions with items in SharePoint lists and libraries. Changes to items in lists or libraries trigger actions in the workflow. Read more in the article "Workflow actions in SharePoint Designer 2010: A quick reference guide" - link
Comparing workflows in the E plans and the P1 plan
Workflows are supported in the P1 plan (Office 365 for professionals and small businesses) as well. Some differences exists when comparing workflows to the E plans to the P1 plan:
Well written blog. Neet and informative.
Informative and audience-appropriate, however there might be some prefatory links that could be included for individuals, like myself, who have used other workflow engines, but don't even know how to enable them in SharePoint Online.
The "source" link
http://support.microsoft.com/kb/2539387/en-us?sd=rss&spid=13163 is broken