Workflows allow to automatically execute a set of actions whenever the defined criteria is met. For example, whenever a new lead is created an introductory email is sent. So this action will keep occurring automatically as long as the condition is true. Automatic actions are triggered upon create, edit or update of a field in the record. In our recent enhancement we have combined field update under edit because update of a value is nothing but edit of a record. Thus, the edit option will now have two options:

• Any field gets modified - The rule will trigger action whenever a field value changes that is, upon edit of a record.
  

• Specific field(s) gets modified - The rule will trigger action when a specified field is modified.
  

For example, if you want to send emails only when the prospects are contacted you can set the criteria as: When the lead status field is modified to contacted send an email. 

• Based on which date/time field the rule should be executed: You can choose any date or time field such as created time, modified time etc.
  
• Define execution date: Specify the number of  Days, Weeks, or Months after which workflow should be executed.
  
• Execution time: Choose to execute at the same time of the activity or enter a time of your choice.