Magento – Difference Between Grand Total and Base Grand Total

Void is in regards to the payment on the order invoice - to void the authorization, for instance - so that the funds aren't subsequently captured. Payments have to be refunded after capture and cannot be voided.

Cancellation, however, is when the order can no longer be modified. While an order payment might be voided a new invoice can always be generated. Cancellation of an order prevents any future change from being made to it.

getOrigData() returns the data in the object at the time it was initialised/populated.

After the model is initialised you can update that data and getData() will return what you currently have in that object.

Have a look at Varien_Object (getOrigData,setOrigData), grep -R "origData" app/ so you can have a look at what is used.

In EE, it's very useful to log what is changed (Enterprise_Logging) module.