Make a difference reads:

E Destroy this foundation: Discard all attacks from both players' card pools. Each player draws X cards. X equals the number of attacks discarded from either player's card pool due to this ability (your choice).

Now when i use this to discard does it cancel my attack? Should it be used on my opponent's turn or mine?

Yes. It aborts the attack. I have used it on my turn and my opponents turn before. I usually use it on my opponents attack (if i cannot block it) to abort it and i usually choose to draw the lower amount of attacks discarded. You could also use it on your attack in hopes of drawing a killer attack.

It should also be noted that you get to choose how many cards you draw via which card pool you choose. For example, on a number of occassions i will let my opponent play out an attack string of really weenie-attacks and take chip damage, and on the final attack i'll activate Make a Difference and choose my cardpool (that doesn't have any cards in it), so no one draws and s/he doesn't net any momentum at the end of their turn, and - oh, look! - they now have fewer cards in hand for my turn.

Stamps. Yall got it exactly right