While working with Gmail the URLs seem to be like https://mail.google.com/mail/?shva=1#inbox. I observed that there is # symbol in URLs. Also Facebook, Twitter will load page content in the similar way without loading entire page it just refreshes required block.
What kind of concept / technology they are using. Can we do so using JQuery? Will search engines can crawl page content in those scenarios?
A jQuery framework that does this is the jQuery.history plugin. It is indeed used (as the comment of marcgg states) by means of AJAX. A tutorial for this can be found here, in its most simple form:
http://www.queness.com/post/328/a-simple-ajax-driven-website-with-jqueryphp
Can we do so using JQuery?
Yes.
Will search engines can crawl page content in those scenarios?
Check @marcgg comment!