Use of apex slds tag
Previously, In order to use of Salesforce Lightning design System either we need to install package or upload files like css, js files to static resources then reference in Visualforce Pages.
Wait! Good news!! In Spring 17 [v39.0] Now we have a <apex:slds /> tag just place tag in Visualforce Page then you're good to use new Lighntning Design System
NOTE:
Don't forget to use div tag along with class="slds-scope" else your styling may not be applicable
Example
<body class="slds-scope">
<!-- HTML/Vf Tags goes here
</body>
For more info on <apex:slds /> refer salesforce link here