From Shopify Wiki
This is the first thing you see when you log in to the admin interface, and for good reason: the Orders tab shows you all the orders your customers have placed so that you can track them, ship them out, and complete the associated payments. Checkout and shipping options are managed in the Preferences section.
When a customer has filled their shopping cart and decides to check out, they arrive at the Shopify checkout page and are asked to provide an email address along with their shipping and billing information. After they've confirmed all the details, a new entry will show up next time you visit the Orders tab.
When a customer checks out using a credit card gateway, the required funds are authorized by the gateway: they are reserved for this transaction, but no money has yet changed hands. Before you receive payment, you must tell Shopify to capture the funds for the order in question. You are expected to actually ship the order to the customer before capturing any funds.
Payments made with PayPal will automatically be captured at checkout and funds are transferred immediately.