Sent means you have sent the message, delivered means it has reached the other persons phone! Depending on app or service, some messages aren't considered delivered if an app isn't opened, or a the phone isn't on!
This thread was the product of our fantastic collaborators during DIGITS Beta! As many terms have changed and bugs have been exterminated since the Beta launched, the information here may no longer be accurate. If you still have a question, please search our active threads or ask a new question in the DIGITS space!