What is the difference between convention and configuration?

1. Configuration - would have been manually mapping controllers to use specific views, almost like a code behind scenario.
2. Convention - A Controller method named Index most likely wants to use a View called the same, so the framework goes and looks for that.

Difference among Order Invoice, Cancel, Void, Hold and Ship in Magento

Pending. - Placing new order. it is default status is pending

Processing - You are raise a invoice from Magento admin status is change to

On Hold - This status can only be assigned manually. You can put any order on hold.

Cancelled - This status is assigned manually by you or, on some payment gateways, when customer does not pay within the specified time.

Complete - This status means that order is crated, paid and shipped to customer.

Closed - Closed orders are orders that have had a credit memo assigned to it and the customer has been refunded for their order.

Pending Payment - This is the status used if order is created and PayPal or similar payment method is used. This means that customer has been sent to payment gateway website but no return information has been received yet. This status will change when customer pays. Some gateways do cancel abandoned orders automatically after some 'idle' time. If order had this status for a long time, you should cancel it manually to free product stock.