Finishing veteran will unlock elite, and finishing elite will unlock ultimate.
You can spend points there to boost your charackter, every cleared shrine in the world will give you one devotion point. Which you should pick in fact is very class / build dependent, you should read some buildguides for your class before spending points there.
1-) I started the game on Veteran Difficulty but Elite and Ultimate difficulties are locked, what should I do for unlock them ?
you have to kill the end boss the Loghorrean
this will unlock the next difficulty
2-) There are name called " Devotion " on skill tree, what is that about ?
it is a extra system that lets you allow to put points (which you get for repairing or destroying monster at a shrine) into a constellations which will give a % proc to a skill you use.
I’ve been wondering, im playing on veteran and can freely switch between normal and veteran having all my riftsites available etc. Can i freely switch between veteran/elite when i unlock elite or does it work like Diablo 2 for example in that i have to redo all the content on elite?