Customer: A customer (sometimes known as a client, buyer, or purchaser) is the recipient of a good, service, product or an idea – obtained from a seller, vendor, or supplier via a financial transaction or exchange for money or some other valuable consideration.

Sometimes a customer can be an organization or group of people.

Adding customer:

To add a new customer go to Customer > New Customer & fill out all the details related to the customer.
Currency: Different customer can have a different currency. So in add customer, you can assign a currency related to the customer.

You can add multiple contacts for a customer. Refer to this document.