What is a thumbnail?
Thumbnail is a small graphic file which is used on HTML pages as decreased image of a graphic file or a video frame. Most of all user clicks on this image to view a video or to open the image in full size.
Creating thumbnails for graphic files is a very simple task. There is a lot of software for graphic files editing. Many applications allow user to create thumbnails for groups of files and this makes the work much quicker. VideoCharge Studio provides the same ability.
When it is necessary to process a video file, there is a number of reasons why this task becomes more complex:
1. The number of applications which can create thumbnails for video files is by an order lower. The number of applications which process groups of files is much less.
2. It is necessary to select a frame for thumbnail: which frame should be used for creating a thumbnail? By default, the application uses the first frame, but what if a user needs the 10th frame? What if users want to select the frames by themselves?
3. It is necessary to create groups of thumbnails (5 or more) for some video files.
4. The last, it is common to create thumbnails consisted of several video frames for video on the torrent sites, adding meta descriptions.
All the described reasons are not a hurdle for the VideoCharge Studio application, one of the best in the represented field. This application has a very wide range of tools for creating thumbnails both for video and graphic files. There is a possibility to automate Thumbnails creating process: it is possible to create several thumbnails at once, process folders with files, work via the command line, find the best frame, and create video at the same time.
Follow the link to find more details about:
Use the step-by-step user guide to simply extract frames from a video file. It is possible to extract either all frames or frames by select query.
http://www.videocharge.com/Products/vcs/doc/extract_frames_from_video.php 
Use step-by-step user guide to create a graphic file. This is the simplest way to quickly and easily create thumbnails for groups of graphic files.
http://www.videocharge.com/Products/vcs/doc/convert_image.php
Use step-by-step user guide to perform video conversion, for example into FLV format, simultaneously with creating thumbnails for each created video file.
http://www.videocharge.com/Products/vcs/doc/how_to_create_thumbnails_for_video.php
Use step-by-step user guide to create several thumbnails for a video file. For example, to create a thumbnail each 10 seconds?
http://www.videocharge.com/Products/vcs/doc/how_to_create_several_thumbnail_for_video_file.php
Use step-by-step user guide to create thumbnail for video file torrent.
http://www.videocharge.com/Products/vcs/doc/how_to_create_thumbnail_for_torrent_video_file.php
Except the described features, the VideoCharge Studio includes a visual interface to give users an ability to select a frame as precise as possible, delete a bad frame, improve the picture applying filters, add watermarks to video or thumbnails, use wizards and so on. And the main thing, the application works with all video formats, folders with files and DVD.

A thumbnail is the term used to describe a miniature version of a slide or picture.  Thumbnail images are created from standard size images and are used on web pages.

Thumbnails are reduced-size versions of pictures, used to help in recognizing and organizing them, serving the same role for images as a normal text index does for words. ..

A thumbnail is the term used to describe a miniature version of a slide or picture.
When working in slide sorter view of PowerPoint, miniature versions of the slides, called thumbnails are shown.

Thumbnail term used to describe a model of slide or image.

A thumbnail picture to a slide or word used to describe a miniature version.When the Slide Sorter view in PowerPoint, the slide is the short version, called the work are shown in thumbnails.

Thumbnail in general term is a nail of the thumb but in technical way Thumbnail is a reduced image of a graphic or document page, used in order to view multiple images on a screen simultaneously or to download such images more rapidly.
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Its shorter or reduced form or image , its used to save space and to paste multi images on one page

The small usually cropped image within an album.A smaller version of the image used to present a faster loading, although other lower-quality formats for larger image, as a GIF or JPEG. This format is not recommended for archival purposes, but can be useful when a large number of images to display on the same Web page.

A miniature display of a page to be printed. Thumbnails enable you to see the layout of many pages on the screen at once. Generally, thumbnails are too small to show the actual text. A reduced image of a graphic or document page, used in order to view multiple images on a screen simultaneously or to download such images more rapidly.

A small graphic, usually included as an inline image link to a larger external image. Thumbnails should be kept small so they load quickly. The user can then follow the links to the larger images, if they wish, and not be slowed down while waiting for the fullsize images to be loaded all at once.

It is a small, reduced size and low quality of image which is used in web page as a clickable button and it is also used as preview of contained images of image galley.

Thumbnail is a undersize vivid record which is misused on HTML pages as ablated representation of a vivid record or a recording frame. Most of all someone clicks on this mortal to compass a video or to unobstructed the individual in afloat filler.

A smaller version of the image used to produce a faster loading, but other lesser-quality image formats larger than the GIF or JPEG. This format is not recommended for archival purposes, but can be useful when a large number of images to display on the same Web page.

small photo, usually included in the link inline image for a larger image outside. The previews are kept low so that they load quickly. The user can then follow the links to larger images, if they wish, instead of slowing down while waiting for the full-screen images can be downloaded at once.

Downsized version of the thumbnail images, to help them understand, organization, services, the role of the same image as a plain text index for words.A downsized version of the image, to show a faster loading, Despite the lower quality, and can replace the big picture formats such as GIF or JPEG format. This format is not recommended for the record, but may be useful when a large number of images must be displayed on the same page.

A smaller version of the image, which is used to provide a faster loading, although of lower quality, the option for the larger image formats such as GIF or JPEG. This format is not recommended for archival purposes, but can be useful when multiple images are displayed on the same page.

According to my point of view there is also some web designers produce thumbnails of HTML code that makes the user’s browser shrinks the image, instead of using a reduced copy of the image. The more thing is that the visual quality of browser the size is usually less than ideal.

Exactly all the answers are meaningful… Even I should say that the thumbnails are the short description of our all kind of files…. This one is also a perfect answer….

A thumbnail is a small, typically low resolution impersonation of an image. It is usually used to display many images on the screen at once. It is attempt to make it easier and faster to look at or manage a group of larger images.

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A thumbnail originally refers to a quick sketch done, jotting down an idea on paper graphically - not something we do much anymore!  - Franciso Bacon