Can I know the different between magento2 stable version which we can download at https://magento.com/ and development version at github. Which can I use for development of new CMS site and to do customizations?
it depends ;)
No really. You can actually also download the stable from github if you choose the latest release for example.
The difference is that the develop branch has all the fancy new features but is far more likely to have bugs or even break the complete shop instance.
So my recommendation would be to start development with the stable version (especially when you are just starting with magento2).
Then if you find a bug or miss a feature you can look it up in develop, it may have already been fixed/added there.
Magento has three types of installations, described here: http://devdocs.magento.com/guides/v2.0/install-gde/continue.html
Which can I use for development of new CMS site and to do customizations?
The "composer" ("System Integrator") installation is the best fit, but if it is too technical for you, choose "download" or ("Easy Installation")
Github installation is targeted at contributors who want to develop Magento itself, not a custom site.