Distinguished between a Federation and confederation motivation of this concept.
Answers
By definition the difference between a confederation and a federation is that the membership of the member states in a confederation is voluntary, while the membership in a federation is not. Sometimes confederation is erroneously used in the place of federation.
There are three terms that are usually used to describe a country's degree of federalism.
Union: It is there least federal structure. India is an indestructible union of destructible units (i.e. states can be destroyed or created by mergers, separation, changing the boundary, etc by the Union but they can't claim sovereignty and get separated from rest of the Union).
Federation: The units are indestructible and so is the Union (or federation, whatever you want to call it). In USA states can't be tempered with, though they can't leave the federation either.
Confederation: The units are indestructible but the Union is destructible, i.e., the units can leave the Union if they want.
But one must note that even these definitions are subjective and I've given the interpretation according to Indian point of view, for multiple nations use the word Union for different purposes (e.g. the Constitution of erstwhile USSR also used this term which was actually confederation).
The other thing to note is that India is not quasi-federal (which would mean that it only looks federal but it's actually not), she's federal. Yes the degree of federalism varies across the world and we've adopted a lesser degree according to our needs.
Third the essence of federalism lies in Division of Power (i.e. states have their own legislative and executive powers unfettered by the Union interventions). I didn't delve into those details for the question pertained to difference between union and federation and not to the definition of federalism.