Thumbnail is a term for a small image representation of a larger image, usually intended to make it easier and faster to look at or manage a group of larger images. For example, software that lets you manage a number of images (Like VidiXplore) often provides a miniaturized version of each image or a video so that you don't have to remember the file name of each image. A thumbnail is also used to mean a small and approximate version of an image or a brochure layout as a preliminary design step. Web sites with many pictures, such as online stores with visual catalogs, often provide thumbnail images instead of larger images to make the page download faster. The user controls which images need to be seen in full size.
Posters are like thumbnails but instead of being a miniaturized version, they are in full resolution, like a snapshot.
Information on how to create Thumbnails och Posters and different query parameters;
How to adjust resolution and quality on Thumbnails;