Linux Kernel Version 3.x

Hi, Please help me understand the difference between Linux Kernel 3.x vs Linux Kernel 2.6. And also i have not understood http://www.phoronix.com/scan.php?page=news_item&px=OTUwMg about 2.6.40 meaning 3.0 version. And also help me understand about linux-next and mainline vs stable branch on http://www.kernel.org/ Please help me understand. Regards, Kaushal