created :

• The workflow rule will be evaluated the rule criteria when a record is created. If the rule criteria is met, run the rule.
• With this option, the rule never runs more than once per record.

created, and every time it’s edited :

• The workflow rule will be evaluated the rule criteria each time a record is created or updated. If the rule criteria is met, run the rule.
• With this option, the rule repeatedly runs every time a record is edited, as long as the record meets the rule criteria.
• We cannot add time-dependent actions to the rule if you select this option.

created, and any time it’s edited to subsequently meet criteria :

• The workflow rule will be evaluated the rule criteria each time a record is created or updated.
• For a new record, run the rule if the rule criteria is met.
• For an updated record, run the rule only if the record is changed from not meeting the rule criteria to meeting the rule criteria.
• With this option, the rule can run multiple times per record, but it won’t run when the record edits are unrelated to the rule criteria.
• For example, Suppose the Director wants to be notified when an Opportunity value is > $50,000. Using this criteria, the rule would fire when the Opportunity is created with a value $60,000 and the Director would get an email notifying him of this Opportunity. Now if the record is changed from a value of $60,000 to $75,000 the rule does not fire again because it has already met the criteria at its initial creation. Now if the reverse happened, and the value went from $60,000 to $40,000 nothing would happen since the record doesn’t meet the criteria for the rule. But, if in another subsequent change, the value increased to $55,000 the criteria would again be met, and since the record didn’t previously meet the criteria during the last change, the rule would again fire and the Director would receive a new alert.