Argentines use the $ sign to indicate the Argentine peso, so unless you see US$, the currency is indicated in Argentine pesos, not US dollars.
You may find it to your advantage to bring extra US dollars with you as some hotels, especially in Buenos Aires, offer a better exchange rate than you’ll find at banks or ATMs.
You may have heard about the “blue dollar” rate and are wondering if you should make use of it. Read more in these articles on Bloomberg and CNN Money.