Difference between CI3 and CI2

Is there a post telling the difference between CI3 and CI2?
If there is, that post should be sticky to the top of this channel ...

There are some improvements and changes in CodeIgniter 3. The most important thing is, that CodeIgniter 2 will reach end of life in October ( http://forum.codeigniter.com/thread-61357.html ). If you are creating a new project, the CodeIgniter 3 is a clear choice. If you have project made in CodeIgniter 2, consider upgrading to CodeIgniter 3. The CodeIgniter 3 should be supported at least several years from now. Both versions are similar. If you will write us what is the reason why are you asking, you might get the more appropriate answer. :-)

Check the user guide, especially the Change Log and the Upgrade notes.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide

Thank you Muzikant and mwhitney.
I asked this question because I learnt CI2 last year while now this year I found that CI upgraded to 3.0.x version, and I wanna know what changed.
I did't read the documentation carefully. As mwhitney pointed out i should read change log.