comparison between edm & grinding

apples and oranges....they are used for entirely different purposes, i wouldn't know where to start to answer your questions

Grinders do parts in seconds and EDM .... hours???

That's the time issue, other then that, I'd say the difference is ... well everything else. Like the post above said. apples and oranges.

Ask me the difference between a bike and a car, both get me from point A to point B, but totaly different.

the best thing you can do is get a good machinist or engineering text book and read it!