When an order is created, either from the storefront or in the Admin, a sales order is created as a temporary record of the transaction. Sales orders can be seen from the Admin by selecting the Orders option from the Sales menu. Payment has not yet been processed and the order can still be canceled.
An invoice is a record of the receipt of payment for an order. Multiple invoices can be created for a single order, each containing as much or as few of the purchased products that you specify. Depending on the payment action, payment can be automatically captured when the invoice is generated.
A shipment is a record of the products in an order that have been shipped. As with invoices, multiple shipments can be associated with a single order, until all of the products in the order are shipped.
A credit memo is a record of a refund. A product cannot be refunded until it has been paid (which means that it has been invoiced). This means that you are not able to create a credit memo before an invoice has been created. The credit memo serves as a record of a refund, but it is not a record of the actual return of funds to the customer.