According to FIDIC (RED BOOK) what is the difference between a "Change Order" and "Variation Order"? What might consist each one of them?

Both refers to the same situation. The use of the term Change order or Variation order depends on what is stated in the terms and conditions of the Contract. A simple way to understand this terms - anything that is different from the original terms and conditions of the contract relating to schedule, cost or specifications. For example, a Contractor was instructed to delete items and requested to add new items this deletion or addition does not have any cost impact. This is still a change order but with no cost implication. 

variation order issued by the consultant engineer for addition or omission regarding the work . change order is an instruction by the engineer to replace defective works, 

A variations clause allows the employer to unilaterally change elements of contract and this ensures ability of contract to respond to practical needs of the project, In FIDIC contract forms change order process does not vary according to the contract forms and

contract types. In FIDIC Red Book standard form; contract provisions related to change order

application have been determined in Clause 13 ‘Variations and Adjustments’ as a part of General

Conditions and in this study the discussions are collected around this clause.

A variation to a scope of work is referred to as variation order or change order. They mean the same and has no distinction in contract application. 

Variation order or Change order are basically same. Their use interchangeably points to the same facts of change in contract,project, content, procedure etc. Variation primarily refers to changes. 

Fidic would differentiate the 2 as:

Variation Order - Changes in contract. (Change in contract documents which place additional commercial or schedule implications on party).

Change Order - Changes due to unforeseen circumstances, drawing deficiencies or scope change.

In my experience a change order is considered as something to be changed within the contract scope, usually by type or amount.

A variation order can be considered as a true variation to contract works either in amount or type and payment is most often based on contracted rates.