I've used these three classes several times by separate several times.
For example when I want to group several animations (i.e.: CABasicAnimations, etc) to happen simultaneously I first think of CAAnimationGroup, when I want to see a layer change from one state to another (i.e.: appear, slide in, etc) I think of CATransition.
But at the same time CATransaction is meant to execute animations in batches (WTH?)
CATransaction and CATransition seem to be different beasts but I usually see CATransitionsinside CATransactions? I've read the docs several times but they are never compared side by side and when/what they should be used for.
It would be great if someone could point what is their relationship/usage :)
CATransaction and CATransition are indeed different beasts...
It seems that the missing bit in your understanding is about CATransaction; once you get that, then maybe all the pieces will fall into place by themselves.
A CATransaction is always created every time you have a Core animation going on.
Every modification to a layer is part of a transaction. CATransaction is the Core Animation class responsible for batching multiple layer-tree modifications into atomic updates to the render tree.
(source)
What happens is that if you do not specify one explicitly, then an implicit CATransaction is created for you.
You can create an explicit transaction (by means of [CATransaction begin/commit]) to tune several parameters of an animation, like whether default animations should be used, how long they are etc. Those are all described in CATransaction reference.
Explicit transactions are particularly useful when setting the properties of many layers at the same time (for example, while laying out multiple layers), temporarily disabling layer actions, or temporarily changing the duration of resulting implied animations.
So, resuming it all, CATransaction is the "big umbrella" under which a core animation animation is run, wether it is a CABasicAnimation, a CATransition, or group animation. It allows you to set some general parameters affecting the way the animation/transition takes place and if you do not provide one, a default (implicit) one is used.
Hope this helps.