Workflows

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. 

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

• Create the workflow to run to "subsequently meet criteria"
• Create a checkbox field called TriggerWorfklow - set it to false. 
• Ensure the Workflow has TriggerWorkflow__c = True as one of the criteria. 
• For each of the records that needs to have the workflow run on set TriggerWorkflow__c to be True. 
• You can do this by importing a list of the ID's and a value of True. 
• Now, when all the records are happily sitting in the Time-Based workflow queue then you can modify the workflow to remove the TriggerWorkflow from the criteria and even delete the field. 
• Or set TriggerWorkflow to be true on all existing records.