Digital stamps are available in a variety of different formats. Two of the most popular and widely used are PNG and JPG. There is one important different between these two file formats and that is that the background on a PNG file is transparent whereas the background on a JPG file is opaque. The easiest way to visualize this is to think of a PNG file being similar to working on a transparency and a JPG as being similar to working on a sheet of paper.
Whether or not this difference matters depends on the project that you are working on. If you are simply printing a digital stamp onto a piece of white paper, for instance for coloring before cutting this out to attach to a project, the difference between a JPG and PNG format won't make any difference. On the other hand, if you want to overlap images then you will need PNG formats. It may be helpful to think of the transparency / paper analogy - the transparency will not obscure any part of an image that it overlaps whereas paper will. The image above illustrates this. In the top illustration, JPG files were used and you can see how the second image partially obscures the first. In the bottom illustration a PNG file was used and the images can be overlapped without any part of the image being obscured.
Note: This illustration also shows one of the weaknesses in digital stamps and that is that it is not easy to replicate simple masking techniques.
If you would like to take a look at the difference between a JPG and PNG file, then you are welcome to download the two files on the following pages.