Windows thumbnail cache
On Microsoft Windows operating systems, starting with the IE4 Desktop Update for Win95/98, a thumbnail cache is used to store thumbnail images for Windows Explorer's thumbnail view. This speeds up the display of images as these smaller images do not need to be recalculated every time the user views the folder.
Windows stores thumbnails of graphics files, and certain document and movie files, in the Thumbnail Cache file, including the following formats: JPEG, BMP, GIF, PNG, TIFF, AVI, PDF, PPTX, DOCX, HTML and many others. Its purpose is to prevent intensive disk I/O, CPU processing, and load times when a folder that contains a large number of files is set to display each file as a thumbnail. This effect is more clearly seen when accessing a DVD containing thousands of photos without the thumbs.db file and setting the view to show thumbnails next to the filenames. Thumbnail caching was introduced in Windows 2000; wherein the thumbnails were stored in the image file's Alternate Data Stream if the operating system was installed on a drive with the NTFS file system. A separate Thumbs.db file was created if Windows 2000 was installed on a FAT32 volume. Windows Me also created Thumbs.db files. Beginning with Windows XP, thumbnail caching and thus, creation of Thumbs.db can optionally be turned off from Windows Explorer View Menu, Folder Options and checking "Do not cache thumbnails" on the View tab. Under Windows 2000, Windows Me and Windows XP, a context menu command to force refresh the thumbnail is available by right clicking the image in Thumbnail view of Windows Explorer.
Thumbs.db files are stored in each directory that contains thumbnails on Windows systems. The file is created locally among the images, however, preventing system wide use of the data and creating additional data load on removable devices. Windows XP Media Center Edition also creates
ehthumbs.db which holds previews of video files. Each thumbnail created in a directory is represented in this database file as a small JPEG file, regardless of the file's original format. The images are resized to 96×96 pixels by default or a proportional miniature of their original shape for non-square images, with 96 pixels on the longer side. The size can be controlled by a setting on Windows Registry. Each folder with initiated thumbnail views (that is where they have displayed a Thumbnails or Filmstrip view in Windows Explorer) will have a Thumbs.db file. Folders with pictures also display previews on their icon when displayed in Thumbnail mode – the first four images in the folder at 40×40 pixels (or proportionally shaped), with a 1-pixel divider overlaid on a standard large folder icon. The Thumbs.db file is stored in OLE Compound Document format, the same format that many Microsoft Office products use.
Centralized thumbnail cache 
Beginning with Windows Vista, thumbnail previews are stored in a centralized location on the system. This provides the system with access to images independent of their location, and addresses issues with the locality of Thumbs.db files. The cache is stored at
%userprofile%\AppData\Local\Microsoft\Windows\Explorer as a number of files with the label thumbcache_xxx.db (numbered by size); as well as an index used to find thumbnails in each sized database.
However, when browsing network shares with write permission, Windows Vista and Windows 7 store a Thumbs.db file in the remote directory instead of using the (local) central thumbnail cache. This can cause issues when deleting remote shares, as the directory will become locked for a period of time when selected as Windows Explorer automatically creates a remote Thumbs.db file.
Creating Thumbs.db files on remote shares can be disabled with a Group Policy setting.
As forensic evidence 
Law-enforcement agencies have used this file to prove that illicit photos were previously stored on the hard drive. For example, the FBI used the "thumbs.db" file in 2008 as evidence of viewing depictions of child pornography.
Sarah Morris and Howard Chivers have investigated the forensic artefacts left by the Windows 7 and Linux thumbnail cache and the relationships that could be formed with the remainder of the system in order to corroborate this evidence. 
See also 
- Thumbs.db files forensic issues: AccessData Corporation
- IThumbnailCache Interface, MSDN, Microsoft Corporation
- Java 2D - Thumbnails Thumbs.db, Oracle Forums
- Turn off the caching of thumbnails in hidden thumbs.db files, Microsoft Support
- Thumbcache Viewer - open-source thumbcache_*.db viewer
- Thumbs Viewer - open-source Thumbs.db viewer
- Thumbs.db Viewer: displays Thumbs.db, Thumbcache*.db (Vista, Win7, Win8) and Ehthumbs.db content (headers of files, entries and thumbnails)
- dm Thumbs - Forensic Thumbs.db and Windows thumbnail cache Database Viewer
- Thumbnail Cache Database Viewer
- Vinetto - open-source tool to examine Thumbs.db
- Thumbs.db file structure details in Vinetto documentation
- Prevent the creation of thumbs.db files via Registry tweak
- Prevent the creation of thumbs.db files via Group Policy (Windows 7)
- Tool that extracts thumbnails from a Thumbs.db file
- Windows thumbnail cache (thumbs.db) description