As we know that there are huge changes in look and feel of SharePoint 2013 while compare to SharePoint 2010. This is all because of Master Page.
There are 2 types of MasterPage which is being used in SharePoint 2013.
Oslo masterpage and Seattle masterpage
On Oslo masterpage left nav is removed and in Seattle master page left nav is available.
Oslo master page uses oslo.css instead of corev15.css