what are the differences between these two?
Price discovery is the difference
If my memory is right, electronic communciations networks work on an auction-type system while crossing networks use order books to match trades at fixed times during the trading day.
My understanding is that ECN is just a alternative type of exchange while you trade normally, 100/1000 shares; while crossing nestwork is specifically for transacting larger share amount, often with buyside desks directly; some crossing network actually are only available to buyside clients. Think of it as the old block desk on the street.