Welcome to new world of Lightning! Hope you all are aware that from Winter'16, Salesforce introduced new UI(User interface) and UX(User experience) for Force.com platform which is lightning experience(Beta version). This future is completely with your control. If you want you can enable the lightning experience or you can use the Saleforce standard UI which 'Salesforce classic'.
Now you might have question,how to enable this future? To enable the lightning experience follow the below steps.
1. Go Setup.
2. Click on 'Lightning Experience' from setup menu.