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.
This process is for void the online transaction, if online payment method is used in order then this process will void current transaction and won't be able to further online transaction process for particular order. (For eg: if some order amount is authorize by third party payment gateway and if Admin apply void process he can't be able to capture payment further more) Although in case of void process order will going on smoothly just all process will only offline(i.e offline invoice, offline credit memo).
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.
This is just order cancellation process. When online payment used with order and amount authorized in this order then. Order cancellation process will call void action for cancel current transaction