Role of 15 Hives in SharePoint

15 Hives is very much imp while discussing about SharePoint.15 Hives is a url as: C:\program files\common files\ microsoft shared\web server extension\15\template\.

15 hives is a directory having overall information while creating a web application in SharePoint

The 15 Hive - Folder Structure

The 15 Hive has a definite folder structure which holds the core SharePoint server files.

ADMISAPI: It contains soap services for Central Administration. If this directory is altered, remote site creation and other methods exposed in the service will not function correctly.
Bin: The directory contains all the core binary files, utilities which used by SharePoint Services. command line tools such as STSADM.EXE also present in this folder.
Client: This directory contains files that are used for creating office apps.
Config: This directory contains files used to extend IIS Web sites with SharePoint Server. If this directory or its contents are altered, Web application will not function correctly.
HCCab: This directory has a set of cab files which has content information used by the SharePoint help system.
Help: The folder contains html help file (.chm) used by the configuration wizard.
ISAPI: This directory contains all the standard Web Services for SharePoint and resources and configuration files that the web services use.
Logs: This is the folder where we can have all the SharePoint related logs will see. This is important when any problem or error occur in SharePoint you have to trace the logs files in this folder to get the error messages.
Policy: This directory contains SharePoint 2013 Server policy files.
Resources: This directory contains the core.resx file used for creating language packs for SharePoint. by which different SharePoint sites with different languages and cultures can be create.
Template: It contains the core web site functionality like the features, templates, configurations, resources of a web site.
UserCode: This directory contains files used to support sandbox solution.
Web Clients: This directory contains files related to Client Object Model.
Web Services: This directory is the root directory where SharePoint back-end Web services are hosted, for example, Excel and Search.

Other Important Directories In SharePoint 2013:

C:\\Inetpub\wwwroot\wss - This directory (or the corresponding directory under the Inetpub root on the server) is used as the default location for IIS Web sites.
C:\ProgramFiles\Microsoft Office Servers\15.0 - This directory is the installation location for SharePoint Server 2013 binaries and data. The directory can be changed during installation.
C:\ProgramFiles\Microsoft Office Servers\15.0\WebServices - This directory is the root directory where SharePoint back-end Web services are hosted, for example, Excel and Search.
C:\ProgramFiles\Microsoft Office Servers\15.0\Data - This directory is the root location where local data is stored, including search indexes.
C:\ProgramFiles\Microsoft Office Servers\15.0\Logs - This directory is the location where the run-time diagnostic logging is generated.