SharePoint 2013 /_layouts/15 vs /_layouts folder differences
In SharePoint 2013 it’s possible to install both SharePoint 2010 and 2013 solutions, which is supported by having two folders – one for the Sp2010 (“14”) hive files and one for the SP 2013 (“15”) hive files.
In IIS a virtual directory called 15 now exists in the _layouts folder of the SharePoint web application. If your legacy SharePoint 2010 solution had assets with filepath references, for example, /_layouts/images/test/test.jpg , you would need to update it to /_layouts/15/images/test/test.jpg in order for the path to be resolved correctly when using the solution in SharePoint 2013.
Now you have to change the css and all the references if you are deploying your data in to 15 hive instead of 14 hive mapped folder.
We recently encountered a commercial web part where the icons for the web part in the Insert web part ribbon bar area were broken. It turns out the developers must have left the old SharePoint 14 hive reference style in and did not make the distinction so the broken image reference had the path:
where it should have actually been:
Follow this link for how to migrate http://social.technet.microsoft.com/Forums/en-US/sharepointdevpreview/thread/93bc1f0a-b84a-41df-9c13-2bdfa92a529f.