Can be, but in terms of foreign nurses they are different. A per diem agency normally does not handle immigration papers, etc. as well as anything having to do with immigration.
Agency work as discussed here means you work for a facility on a day to day basis in most cases, sometimes can be a contract for acertain amount of time, but they do not pay your housing, etc. When I still lived in Michigan, I used to contract for several months at a time in an OR, but that was still "agency" and not a travel contract.
Hope that this helps clear up some things for you.
The agency that you have contracted with, more than likely will place you with one facility for the length of your contract and either they will handle your immigration documentation or the hospital will.