Thumbnail - a small-sized, concise, and compact representation of images or videos. Such reduced copies are a common practice in any programs and web applications for working with graphic files (raster, vector, fractal) and video-records.
In this case, thumbnails can be widely used for any information that the user is accustomed to perceive visually (CAD drawings, 3D models, a screenshot of the current place in the save in the game, etc.).
When working with video, thumbnails are also often used. In the video editor on the timeline, a user can display reduced copies of keyframes to navigate faster. If this is a software application for watching videos, then for the presentation you can automatically or manually select a thumbnail for an image. Some programs and Web services generate several thumbnails for one video at a time. When viewing a set of videos, they display only one of them, but when you hover the mouse cursor over a thumbnail, it is quickly replaced by others so that the user gets a better idea of the video that interests him.
Thumbnails usually have only a descriptive function. Therefore, in order to save resources, without significantly poor resolution and damage, they can be saved in less quality than the original one.
Thumbnails also do not have to be stored in the same format as the original image. The developer can choose some uniform format for everyone to simplify the task. Based on the same descriptive function, for a thumbnail you can not use the whole image, but only its fragment. Often this can be even more informative and useful. Some images turn into an uninformative pixel mush (for example, schemes), and then it is clearly better to use the recognizable fragment.
Each manufacturer of a software product usually determines the fixed maximum sizes of thumbnails (usually the same for width and height). To create them, the images are proportionally reduced so that the larger size does not exceed the specified limit. This simplifies the design of the user interface, as the developer is confident that the picture does not go beyond a certain size.