Your account has one primary currency, which should be the same currency that your bank account is in. It is the currency we use to calculate your revenue in all the places where that is shown.
In addition to this, you can add as many additional currencies that you can quote prices in, as you want.
Both are under Settings > Account.
On the checkout form, your customers will have a choice of currency and will see the prices in the currency they choose.
You can also set the default currency for a product, so if you have something that is targeted toward a particular audience that uses a particular currency, you can default the order form to that currency.
In addition, when paying, customers can pay in pretty much any currency, and their bank or PayPal or whoever will do the currency conversion for them.
If you are running on international business check out this guide for more support.