As I understand, the economy mostly runs on gray market US currency (gray market because the Argentine government doesn't acknowledge real exchange rates and sometimes takes capricious action against US currency use.)
There is the official exchange rate set by the government and a number of other rates for various purposes.
The two of relevance to someone visiting from the US on vacation are the "blue dollar" (a black market exchange rate that's basically double the official rate) and the MEP rate (exchange rate used by some, but not all, purchases on American credit cards).
