What does the "Processing" status mean (processing payment, shipment, ...) and where can I find a complete list of Magento statuses and their definitions?
Take a look at http://www.mexbs.com/magento-order-statestatus-flow/
Order "Pending" From the Admin panel, the sales order appears in the Order list as "Pending". At this point, payment has not been processed, and the order can still be canceled.
Payment Received The status of the order remains "Pending" until payment is received. Depending on the payment method, you, may be notified when the transaction is authorized and in some cases, processed. The status of the order is now "Processing" and order can not be cancel and you will need to issue a credit using Credit Memos see http://www.magentocommerce.com/knowledge-base/entry/order-process-overview
Angry Goomba,you can see list of status at
an Order state have multiple Order status and One default status.
If you have set
a order state and it also changed the order status. it default Order status
Processing means The
order status is Processing and it state also Processing