C:WindowsCSC folder

C:WindowsCSC folder is the one where Windows keeps the cache of the files and folders for which offline files feature is enabled. Offline feature is present in Windows 2000, XP, Vista and Windows 7 OS editions.

Windows treats this folder as system file and does not display them in default configuration. To see this folder you need to open Explorer -> Tools -> Folder Options -> View and then select the check button ‘Show hidden files/folders‘ and unselect the one ‘Hide protected operating system files’.
 The structure of the folder varies based on the operating system. In Windows XP, CSC folder contains folders named d1, d2, d3, d4, d5, d6, d7 andd8 and also four files namely 0x00000001, 0x00000002 and 0x00000003 and csc1.tmp

These directories contain the copies of the files of the network share which is offline enabled and the four files store the metadata information about the cached files.

This CSC folder can be moved to some other location by editing the registry key DatabaseLocation under the following node.
HKLMSoftwareMicrosoftWindowsCurrentVersionNetCache

In Windows Vista and Windows 7 C:WindowsCSC folder has a different directory structure.

C:WindowsCSC has occupied lot of space on my disk and is causing low free space issue. Can I clean up this folder?

You can delete the files but you need to check first whether any users of the system have any files on the PC which have local changes that are not yet synchronized with the server copies. If not, then you can delete the cache using the methods mentioned in How to delete offline files cache in Windows XP.

For Vista and Windows 7 see the below post.
How to delete offline files cache in Vista or Windows 7