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. Art directors and graphic designers use the term "thumbnail sketch" to describe a small drawing on paper (usually part of a group) used to explore multiple ideas quickly. Thumbnail sketches are similar to doodles, but may include as much detail as a small sketch. I can't stress the importance of thumbnails, this creative process makes you push yourself and not relied on your fist option. It is also important to save them and see how did you resolved past problems. I love this process of the evolution of the ITunes logo and Drawger always have great examples. Take a look of my thumbnails here, here and here. There are also amazing books about sketches/thumbnails from the pros.
images via: drawger