The difference carries over from the respective differences of the prepositional objects the verb freuen can take. The dar- forms are merely demonstrative pronouns to avoid repeating the same noun.
Sich auf etwas freuen
This does not mean to be delighted about something. Its only meaning is to be excited about something, something necessarily being a future event.
Sich über etwas freuen
This is the phrase you are looking for for to be delighted about something.
Compare the following examples:
Ich freue mich auf das Eishockeyspiel.
Morgen ist das Eishockeyspiel. Ich freue mich darauf.
The ice hockey match is tomorrow and the speaker is looking forward to it. Note the demonstrative preposional pronoun darauf in the second example.
Ich freue mich über das Eishockeyspiel.
(It’s a little bit harder to generate a sentence wherein darüber clearly refers to the game and only the game. I expect suggestions to appear in the comments or edited in accordingly ;-))
Here, the match has happened. Something about the match was so great, that the speaker is delighted about the match as a whole.