An additional currency becomes available to products, shipping rates, and markets once it is added and enabled in Settings > Currencies. Markets decide which currencies customers can use in each country.
Where you enter a price for a currency, customers pay exactly that price. Where you don't, the price is converted from your base-currency price using that currency's exchange rate and rounding, and customers pay the converted amount.
Orders are recorded in the customer's currency, with base-currency amounts fixed when the order is placed.
Gift Cards you issue yourself must use your base currency or another enabled currency. Gift Cards bought in an order keep the order's currency.