By definition at Merriam-Webster Online:
thumb·nail noun [thuhm-neyl]
1. the hard covering at the end of a thumb: the fingernail of a thumb
2. computers: a very small copy of a larger picture on a computer
Since we all understand the first definition, let’s talk computers. A thumbnail image is a tiny graphics file. It is a small-scale, low-resolution image generated from the original image. Thumbnail images are created from standard size images and are used on webpages. The small size allows Web surfers quick access to webpage content. People would find retail shopping nearly impossible if it weren’t for the use of thumbnail images, and surfing pages with multiple images would simply take too long.
In many cases, thumbnail images are clickable, causing a larger image to load at the user’s discretion. This makes webpages user-friendly, saving the user from having to download large image files that he or she doesn’t need or want to see. With a page of smaller images, the user can click on only those images of interest and then download the large, original, high-resolution image file and not the small, low-resolution thumbnail image.
So, when designing your print documents, be sure to download the high-resolution image and not the thumbnail for the best quality image.