Overview

Trailhead has a nice overview of Workflows https://developer.salesforce.com/trailhead/force_com_declarative_beginner/business_process_automation/workflow including this video

Practically, "created, and any time it’s edited to subsequently meet criteria" is the most important part of Workflows to know about. 

You will ALWAYS forget to Activate your Workflow - ALWAYS. It's guaranteed! 

See Also

Process Builder, which is not ready for prime time yet, IMO. 

Triggering Workflows to run on existing records

https://success.salesforce.com/answers?id=90630000000gmtfAAA