Yep, you can use it whenever you want.
Express line = Fastpass line (pretty much, although I would guess that the wait times in the Express line would be a little more up and down since there aren't a limited number of tickets for each time slot)
If you stay onsite, you can use the Express line all day long as many times as you want by showing your room key.
If you buy the Express Pass, you can use the Express line ONCE for each attraction, but you can use it whenever you want during that day. There is no time window. I thought the prices ranged from $25 to $45 or so depending on how busy they expect to be.
Think of it this way. Imagine that Disney took away the Fastpass machines, but sold a book of Fastpasses that included one pass for each attraction with a time window that went from open to close on the day of purchase. That's the Express pass you can buy at Universal.