What are the main differences between cakephp 2 and 3?

Cakephp 2

CakePHP is a rapid development framework for PHP that provides an extensible architecture for developing, maintaining, and deploying applications.
Using commonly known design patterns like MVC and ORM within the convention over configuration paradigm, CakePHP reduces development costs and helps developers write less code. ”
And now, it even got better. After a year and a half of coding, CakePHP 2.0 is released.
New features of Cakephp 2
 
Use of native features like exceptions, PDO, SPL, json_encode and much more.
Embraced the PSR-0 recommendation for file naming. Now all your classes map to a file with the same name. Less things to remember!

New Error and Exception handlers provide easier to configure, and ease working with errors such as page not found, unauthorized error and lots more.
New Request and Response objects for easier handling of HTTP requests.

Brand new email library with support for multiple transports. With easy to   use global configuration, logging, and charset support.

Reworked the Security Component to suck less, now you can more easily protect your apps against CSRF and form tampering attacks while having fun.
HTML 5 form inputs support in form helper.

A Sexier default look taking advantage of new CSS 3 features.

A lot faster, almost everything is now lazy-loaded, and even on debug mode you will feel your applications flying.

CakePHP 2.0 completely dropped the PHP 4 support and is now supporting PHP 5.2.6 and above. This was expected.

Cakephp 3

Let us look out some major features of Cakephp 3
ORM Improvements
Numerous API changes are encountered by object-relational mapping (ORM) to make much simpler to acknowledge and authorize associations with saving operations.
Enhanced components and helpers
    A new Version 3 released enables the Flashhelper and FlashComponent  to provide enhanced support for advanced select technology parameters to be involved in development of “flash messages”.
Best proficiency
    Version 3 ensures high performance for reverse routing process, which goes beyond the limit of the framework itself. CakePHP 3.0 offers a flexible caching layer that assimilates improvements to the bootstrap process, routing process and numerous parts of the process for generating helper templates that provide a strong indication for attacking the issues to developers using the framework.
Improved Session Management
    Here after the necessity of managing sessions in CakePHP through a static class is not required. Now, the sessions can be accessed through the request object “$this->request->session ()” to call, evaluates and controls it in various routes by using the new version. This process undergoes with better cross-session response within the process by allowing CakePHP to use PHP unit4.x.
Improved consistency of conventions
    It is a fascinating aspect that enables to retain the same directory structure for the application and plug-in skeletons that are updated to make it grievance to the entire system, which gives more consistency in proficiency wise.
Improved bug-fixing tool
    Version 3.0 has been integrated with bug-fixing tool that provides turnkey solutions for solving new application errors rapidly and easily during development process stage.