What's the difference between dramatic and proleptic irony?

Both of these are literary devices used in playwriting. Dramatic irony occurs when the audience knows something relevant to the plot that the characters are not aware of. For example, in a play about the assassination of JFK, the audience would be expected to know that the president is going to be killed, and how it happened. The characters in the play, however, are not aware of this. Proleptic irony, on the other hand, occurs when an earlier event gives the audience a clue ("foreshadows") a later event in the play. To use the scenario of JFK's assassination again, perhaps a security advisor might say to the president earlier in the play that it would be worthwhile increasing the security detail when the president is in public, as there is an increased threat of assassination. This would foreshadow the actual assassination of JFK – it gives the audience a clue that perhaps JFK is not being cautious enough, and that there is a chance he will be killed.