Welcome New Twitter

Twitter is planning to migrate all of their users to the new site after testing it for a few months. For some time, the users have their options to revert back to the old version of Twitter, but within a week from now, those options will no longer be available, as Twitter will migrate it automatically and the old version will be just a good bye..
If you haven't tried the new version, it's your chance to take a look and get used to it because sooner or later, that's the web interface you will be seeing everyday if you don't use any Twitter client apps.