Some of the folders in 14 hive are :

Program FilesCommon filesMicrosoft SharedWeb Server Extensions14 –
This directory is the installation directory for core SharePoint Server files.

Program FilesCommon filesMicrosoft SharedWeb Server Extensions14ADMISAPI –
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 FilesCommon filesMicrosoft SharedWeb Server Extensions14CONFIG –
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 FilesCommon filesMicrosoft SharedWeb Server Extensions14LOGS –
This directory contains setup and run-time tracing logs.

Other newly added folders are :

Program FilesCommon filesMicrosoft SharedWeb Server ExtensionsPolicy –

Program FilesCommon filesMicrosoft SharedWeb Server ExtensionsUserCode –
This directory contains files used to support your sandboxed solutions.

Program FilesCommon filesMicrosoft SharedWeb Server ExtensionsWebClients –
This directory contains files related to the new Client Object Model.

Program FilesCommon filesMicrosoft SharedWeb Server ExtensionsWebServices –
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.

See more details @ 14 hive and other SharePoint 2010 directories