Microsoft has replaced the "12 hive" structure that we had in SharePoint 2007 with "14 Hive" structure in 2010.
Some of the folders in 14 hive are :
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\14.0 - This directory is the installation location for SharePoint Server 2010 binaries and data. The directory can be changed during installation.
C:\ProgramFiles\Microsoft Office Servers\14.0\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\14.0\Data - This directory is the root location where local data is stored, including search indexes.
Program Files\Common files\Microsoft Shared\Web Server Extensions\14 -
This directory is the installation directory for core SharePoint Server files. This directory is the installation location for SharePoint Server 2010 binaries and data. The directory can be changed during installation.
Program Files\Common files\Microsoft Shared\Web Server Extensions\14\ADMISAPI -
This directory contains the soap services for Central Administration. If this directory is altered, remote site creation and other methods exposed in the service will not function correctly.
Program Files\Common files\Microsoft Shared\Web Server Extensions\14\CONFIG -
This directory contains files used to extend IIS Web sites with SharePoint Server. If this directory or its contents are altered, Web application provisioning will not function correctly.
Program Files\Common files\Microsoft Shared\Web Server Extensions\14\LOGS -
This directory contains setup and run-time tracing logs.
Other newly added folders are :
Program Files\Common files\Microsoft Shared\Web Server Extensions\Policy -
Program Files\Common files\Microsoft Shared\Web Server Extensions\UserCode -
This directory contains files used to support your sandboxed solutions.
Program Files\Common files\Microsoft Shared\Web Server Extensions\WebClients -
This directory contains files related to the new Client Object Model.
Program Files\Common files\Microsoft Shared\Web Server Extensions\WebServices -
This directory contains new wcf or .svc related files.
Note : You should rewrite and recompile any code that refers to files and resources in "12" Hive structure.For example, if you have redeployed all of your files into the "14" folder and emptied your "12" folder, any references to files under the "12" folder will not work.