are the terms 'alternative universe', 'alternative timeline' and 'parallel worlds' the same thing or are their differences in these ?

They are all science fiction. None of them has any scientific definition. 

Alternate usually is a reference to possible but didn't happen. 

Alternate universe is open to how things might be under some theoretical conditions. 

As used in the popular culture, these terms are unscientific, meaning that an idea or theory is so far out that it's in another universe of ideas. However, those on this forum who say there is no such thing scientifically, aren't acquainted with the relevant scientific theories. There are indeed valid scientific theories that there are other universes, one of the theories being that there are parallel universes, and another that there are bubble universes. The idea of these theories is to account for the beginning of our universe, since many physicists aren't comfortable with an unknown situation at and before 10^(-43) seconds which is where knowledge of the Big Bang stops because  the math of quantum mechanics and relativity collapse at that time.

just trying to legitimise  it , theres no such thing as a parallel universe you fool .its made up for the movie people .

An "alternative universe" or timeline is connected to OUR own in some way---Like ours but either a branch off or near copy of our own with some differences.    A parallel UNIVERSE is an entirely different UNIVERSE, with it's own stars, planets and galaxies existing in the same effective "Space as ours" but at a "different "frequency" so to speak so we can't interact with it or know it is there, and vice versa relative to the other universe too.   

The alternative universes are at a different frequency of reality too, but they were once OUR universe before splitting off into its own parallel existence.-----So all these universes share the same space but are only "REAL" relative to matter with their same Wave attributes.

MATTER is ultimately WAVES, and the only matter that is real to other matter has the same wave attributes.----So many many different universes can exist in the same space being REAL only to itself.

There is no "Solid" matter as such--solidity is only an illusion of opposing forces within a given universe.----Folks who think of matter as "little HARD balls" are pre 1920s in terms of their Physics.

As used in science fiction there are differences.  Parallel worlds were never connected.  Alternative timeline was the same as ours to some defined point, and from then on was different.  Example is Germany winning one of the world wars or Booth's gun misfiring.  Alternative universe is a more profound change involving stuff from off-Earth, such as Mars invading New Jersey in 1938.

Well... depends on your philosophy. 

An "Alternate Universe" is one that is completely different from ours... while ours continues to unfold the way it does, an alternative universe could move to the future in a completely different manner;  while the two may begin exactly the same, they may end up diverging and result in vastly different outcomes. 

An "Alternatve timeline" may refer to a single universe;  in Sci-fi, it's common that someone time travels to the past, changes something, and now the future they came from is slightly to vastly different...  Like, "Back to the Future", Marty McFly changes events that lead to how his parents met, and who Biff was - and, returning to the future, he sees how those minor changes made an impact.  Or, in Star Trek's "City on the Edge of Forever", McCoy goes back to 1930, and saves a woman from dying - which completely changes the future of Earth.  

Depending on your view (it's really just a mind experiment) - either the two timelines, the original and the new one - unfold together (which means you probably have a second universe for both to be true), or the original timeline is destroyed, replaced by the new (altered) one. 

"Parallel worlds" (or parallel universes) is pretty much the same as an 'Alternate universe'.  There's a theory that there are *many* universes that exist; Some where there's no Earth at all, and others where the differences between *this* Earth and *another* Earth are so small you may not be able to tell of any between the two. 

All the same. All fantasy.

The terms tend to be used interchangeably but there are differences.  An alternative universe could always have been discernibly different from this one.  For instance, the strengths of its fundamental forces could have been incompatible with the formation of heavy elements.  An alternative timeline would be one which either branched from ours with at least one point of departure (e.g. a decision made on flipping a coin) or became discernibly different at some point in the past.  A parallel world would be the same as an alternative universe.

There are at least three different kinds of alternative universes.

In an infinite universe - which observational evidence and general relativity seem to imply - all possible combinations, including duplicates, will exist in different regions of the infinite universe. You can actually compute how far away a duplicate region would be ...

In the standard multiverse theory, there are countless universe embedded in a ‘bulk’ universe. These would be causally disconnected from each other.

In quantum mechanics, the many-worlds interpretation posits that all possible outcomes of quantum superpositions occur in causally disconnected threads.

Alternate timelines could be given to the notion of the many-worlds interpretation, but they really aren’t alternative timelines. Its really just a science fiction artifact, generally of some sort of time travel, where intrepid time travelers can cause branching of timelines to prevent paradox.

There is really no evidence that any of the above actually exist.