Subpoenas are orders, but they're not necessarily court-issued. Warrants, on the other hand, are court-issued -- the police can't issue warrants on their own in the US.
I realize this comment is a little ambiguous: the order in the warrant case is an order by the court to the court's officers to perform an arrest, seizure, etc. It's not an order for you (the subject of the warrant) to comply.
